I think the original poster does have a bit of a point here. If a hostel does in fact advertise as "free" and "donations accepted" then they can't complain if someone does stay for free and does not donate money. If the hostel owner expects donation or work for stay, then advertise that and then you have a gripe when someone doesn't donate or work for stay.
There are plenty of people, young folks especially, that don't have the funds and a free place to stay is a great thing. The ones with more money will hopefully make up for them by donating extra.
Of course, in this case, I do not know if the original poster actually acted like a "scumbag" or JUST didn't make a donation. Every story has two sides.