A fair point, Lancer. I think something to think of when keeping a long view of things is what purpose volunteering can have. If you need to get food in your belly, you certainly have less leeway in where you distribute your time, and a paycheck is a necessary priority.
But if you can find free time around work, or free time around your schooling, volunteering to teach is a wonderful opportunity. It essentially becomes a resume item equivalent to an internship or entry level job. Furthermore, volunteering is a level of generous sacrifice that mean a lot to whatever organization you volunteered with.
It’s very easy when you’re a student to assume it’s a two-step process of #1. go through school and make my portfolio, and #2. have job. But many people then discover the catch 22 of needing more experience to find work, but needing work to gain experience. Volunteering to teach somewhere is just as solid a resume spot as an internship, and either of these can serve as the almost mandatory stepping stone between #1 and #2.
IMHO, don’t ever think of it as getting taken for granted; think of it as the opportunity that’s going to lead to the better opportunity. Hope I don’t sound too preachy, but “getting taken advantage of” in the exact way you described, and embracing it, helped lead to more legitimate opportunities down the line.