If I remember correctly, Amazon has sued over web-only supposed patent infringement. It's just that Amazon is so big and powerful, while Yahoo has fallen from grace. I hate it when companies feign moral outrage over what is simply corporate warfare. There are very few companies that, when standing over a cliff like Yahoo is, would choose the right thing over using whatever is in their arsenal to survive. So, yeah, changing the rules of the game is a necessity, but it is Yammer's CEO who's crossing the line by implying Yahoo employees are somehow tainted, like ex-Nazis or something.
One company does stand to (possibly) gain from all this - Google.
Amazon did get a lot of flak back then, there was a boycott. And Amazon, thankfully, stopped its lawsuits. Meanwhile we have seen an entire industry - mobile - become a patent battleground, so people are more concerned now then then.
I do see the point that threatening employees to leave or be blacklisted is... disturbing. I think we should put pressure on Yahoo and its employees to change Yahoo's ways, but not like that.
Edit: Google doesn't stand to gain from this. Well sure, in the short term Facebook's loss is Google's gain. But if patent lawsuits become a norm in the internet industry, Google in fact has perhaps the most to lose.
One company does stand to (possibly) gain from all this - Google.