Some people are probably just upset because of an irrational sense of entitlement ("I paid for X, and now I won't get free updates forever as the product is being abandoned")
But there's lots of other reasons that people can be bothered by this.
I use the Mac version of Sparrow (without using Gmail), and the entirety of my reaction when reading about the acquisition was "Oh well, I guess at some point I'll have to find a new desktop email client. That sucks, I liked Sparrow."
I do wonder what the community reaction would be if it was a different product though.
If Sublime Text (which seems to be attracting a good following) announced tomorrow that they were being acquired, and that the product was being abandoned, would people complaining about it really be that surprising?
Even assume that they were also going to continue with maintenance on it, but stop active development of new features. People would be upset. People on here would say "You should never have assumed that Sublime was going to get new features before buying it, if it didn't do what you wanted now, you shouldn't have paid money for it" (I've found myself actually giving similar advice in the past regarding buying smartphones).
People would write asshole-ish blog posts saying that those people were whining.
I don't think this is actually an indictment on the "paid vs. ad supported" dichotomy. Other people have rightfully pointed out that companies get acquired like this all the time, regardless of whether their products are free, cheap, or expensive.