The claim isn’t that “you cannot stop”, but that it’s rude to not communicate about that ahead of time.

Of course the entitlement to volunteer work is also rude, and in my opinion worse.

This is exactly what happens when you volunteer. When you've had enough, or just want to spend your time in other ways, you're hounded to come back, to continue to help, and to varying degrees made to feel guilty because you decided to stop doing something that you had been offering for free.

I don't think this is a reason to never volunteer but you have to develop a thick skin, know where your lines are, and at some point politely but firmly say "no."