Why does Keto allow artificial sweeteners?

Artificial sweeteners by themselves doesn't cause an insulin response in any studies I've seen, but it might be that people have conditioned themselves to get one from artificial sweeteners. So it's possible that some people might need to be careful around artificial sweeteners while "unlearning" the response, but for the vast majority there shouldn't be any insulin related issues.

Mark has a more thorough article, "Do Artificial Sweeteners Cause an Insulin Spike? ".

From another thread:

Seems like you can probably get an insulin response if you've conditioned yourself to it.

Cephalic phase responses can also be stimulated by Pavlovian conditioning; i.e. animals can learn to associate arbitrary sensory stimuli with the availability of food, and then will react as if the food itself has been perceived. For example, insulin secretion in anticipation of feeding can be associated with other environmental stimuli, such as time of day, sounds, visual cues, or tastes (e.g. Woods et al., 1977). Rats fed at a certain time everyday begin to secrete insulin time locked to a circadian clock (Woods et al., 1970; Dallman et al., 1993).

"Anticipatory physiological regulation in feeding biology: cephalic phase responses", Power ML, Schulkin J. Appetite. 2008 Mar-May;50(2-3):194-206. Epub 2007 Oct 24.

It's in animals, but skimming abstracts from other studies says the same for humans:

Reviews studies considering whether hyperinsulinemia and its resultant effects on adipose tissue mass can alter perceived hunger, taste, and food consumption.

...

They show that high acute levels of insulin can be produced by simply seeing and thinking about food and that individuals showing this response show a greater tendency toward weight gain in a food-abundant environment.

"Insulin levels, hunger, and food intake: an example of feedback loops in body weight regulation". Rodin J. Health Psychol. 1985;4(1):1-24. (no free source)

The conditioning should wear off after a while (or it might have already worn off) if you don't keep reinforcing it by eating when you get the urge. You have more stable blood sugar on Keto which helps with self control, which might add to why you have an easier time controlling it now.

And from yet another thread:

From what I can find it seems that you are right that sweeteners doesn't give an insulin response. Studies quoted are for Aspartame, Saccharin and Sucralose (Splenda).

Twelve normal subjects and 10 subjects with non-insulin-dependent diabetes mellitus were given, in random order at intervals of greater than or equal to 1 wk, three drinks of the same beverage: one unsweetened, one sweetened with 400 mg aspartame, and one sweetened with 135 mg saccharin. The amount of sweetener approximated that in 1 L of sugar-free soft drink. Plasma glucose, insulin, and glucagon were measured for 3 h after ingestion of the test beverage. Plasma glucose declined slightly throughout the test period, probably due to fasting, with no differences between the three treatments. Neither sweetener affected peak insulin levels in subjects with or without diabetes.

"Response to single dose of aspartame or saccharin by NIDDM patients".

Because large doses of phenylalanine stimulate prolactin secretion in man, we studied the acute effects of oral doses of aspartame (0.534 g, equivalent to the amount of aspartame in approximately 1 L beverage), aspartic acid (0.242 g), and phenylalanine (0.3 and 1.0 g) on serum prolactin and other hormones in normal humans. Prolactin was not stimulated by any of the aspartame meals, aspartic acid, or 0.3 g phenylalanine; a small rise in serum prolactin, similar to that produced by a high-protein mixed meal, followed ingestion of 1.0 g phenylalanine. Serum growth hormone showed no statistically significant changes in response to any of the experimental meals whereas cortisol and insulin fell slightly and glucose rose slightly during each of the meals. We conclude that these doses of aspartame do not alter secretion of prolactin, cortisol, growth hormone, or insulin in normal individuals.

"Aspartame and its constituent amino acids: effects on prolactin, cortisol, growth hormone, insulin, and glucose in normal humans". Emphasis added.

METHODS: Fasted subjects were studied on 4 study days in random order. Subjects consumed 50 ml of either water, sucralose (0.083% w/v), a non-sweet, glucose-polymer matched for sweetness with sucralose addition (50% w/v maltodextrin+0.083% sucralose) or a modified sham-feeding protocol (MSF=oral stimulation) of sucralose (0.083% w/v). Appetite ratings and plasma GLP-1, PYY, insulin and glucose were measured at regular time points for 120 min. At 120 min, energy intake at a buffet meal was measured. CONCLUSIONS: At this dose, oral ingestion of sucralose does not increase plasma GLP-1 or PYY concentrations and hence, does not reduce appetite in healthy subjects. Oral stimulation with sucralose had no effect on GLP-1, insulin or appetite.

"Effects of oral ingestion of sucralose on gut hormone response and appetite in healthy normal-weight subjects". Emphasis added.

Just revised Wiki, looks like some researches do conclude that artificial sweeteners could increase insulin level. However, insulin is not bad by itself, high blood sugar level is.