Thats the tradeoff you have to deal with if you want to use a free service.
I think the bigger issue here is the entitlement that seems to come with every service online now days. People seem to think they are entitled to demand whatever they like and how they like it for things they aren't even paying someone for. I think the best example of this was all of the whining about Craigslist suing Padmapper because it was ripping off Craigslist content and trying to piggyback off of the work CL has put into building their brand and infrastructure.
He is entitled to be outraged, stop it.
"I just found out that this company was infecting all their users with bubonic plague."
"Stop complaining. It was free!"
Offering something free doesn't make one immune to criticism and criticizing isn't entitled whining. This is fair criticism.
Stasis is good. Let some other site be the one to "innovate".
I think it is more of a feeling of breaking a "gentlemen's agreement". When a free service provides a service for X amount of time (expectation building), and you provide them with lots of data and additional users (network effect), and then suddenly they change the rules, it is upsetting. Being a victim of bait-and-switch is not a pleasant feeling.
By all means, they have the right to shut down their service, or change their policies, etc. But I think their underhanded way of implying the service-user endorses something which they don't is ethically wrong whether there is an existing business relationship or not.
In summary, I feel there is a set of things I absolutely have the right to demand of free services. There is a different set of things which I would prefer they not do, but are completely within their rights (shutting down the service, charging for it, changing the way their comment model works, anything like that).