Sites don't sell often, but when they do, the seller is entitled to it all.
In your humble opinion, and making certain assumptions about the scale and purpose of contributions.
Legal ownership is never everything. If you buy a house, I move in, together we do it up, but you pay for most of the materials - then you sell. Its most definitely not a fact that I am entitled to nothing just because it was yours to sell.
Whether anyone can be bothered is a totally different question, and all will depend on the facts in any given case, but it would be fascinating to know what a court would make of a claim from overworked unpaid mods on a forum sale one day.
I can see why long term users could feel let down
P.s. this is idle chat about the concept of sole ownership vs common enterprise with regard to forums in general - I dont know all the facts on this one. The one I was involved in before the owner repeatedly said was not for sale, asked for and got financial help, then sold. I could understand his reasons, but again could see how some may have felt cheated.