He has his reasons too -- trying to chip away at the defacto monopoly that MS has in the market place. Will it make any difference? Hardly, but any bit can help. Merely letting people know that alternatives exist can be a reason.
And they come across like a really annoying friend who constantly keeps trying to get you to watch the Star Wars Trilogy on his big screen TV, even though you have your own perfectly good big-screen TV. That's basically what they achieve in doing that: they come across as annoying and pushy. I don't mind a Firefox button on a site, I've no problem with them basically advertising another browser.
I do mind sites that tell me I shouldn't be using IE because they don't like Microsoft's monopoly. I'm not fond of Microsoft's monopoly either, but I can credit people with the intelligence to decide for themselves. I can also accommodate users who may have no choice but to use IE, if they don't have the permissions to install their browser of choice.