Yes. However, invalidating patents via prior art takes long, and is quite expensive. Which is precisely why Lodsys is targeting the people they are -- if they tried to go after any of the big guys, they'd lose their patents. The small guys can't fight back.
It's not that they always chicken out, sometimes it's just cheaper to not fight. Which often is the strategy in these types of lawsuits, make an offer to settle that's less than the cost of litigation. This has gotten so bad lately that many letters are simply bold, in-your-face threats of extortion that for some unknown reason the courts have done little to address.
Plus, for some of the bigger companies you say chicken out, this is a potential winning strategy for them to stifle competition. The big companies have the funds to just settle and move on while the small companies that might pose a threat in the future may have a harder time dealing with even just a simple settlement.
