Doing work without being payed?

I work at a small family business, specifically a specialty grocery. I am happy with my job, although there are a few practices that have made me raise an eye (then again this only my first job so perhaps these are more common than I'd like to believe). For instance, certain employees consistently work off the clock (think 10+ hours/week).

Recently I was asked to write an article for their website, one that would be a historically informed, well written piece. In other words, not a short advertisement but a real thing of writing that would require hours of research and editing.

Initially I was flattered to have been asked, yet the more I think about it, I wonder if I'm finding myself in a similar position to some of my coworkers. There was no talk of any compensation for doing work that is at the benefit of the company. Does this constitute unpaid work, or am I just being oversensitive?