Confusion about being hired as on demand

I was hired as on demand and am reaching my 90 days soon. In the beginning, HR told me I'd be regularly scheduled during the 90 days, so I was under the assumption that I'd stop being scheduled after that. However, I checked my schedule and saw that I was still being scheduled after my 90 days, and I had already picked up another job so I can't make those shifts. I contacted HR about it, but they said it's up to the ETL and my standing (like attendance) to see if I can go on demand. Is this normal or am I not understanding this?? Idk why I have to request to go on demand when I was hired as one?