Unfortuately I think Microsoft had to add that due to some legal agreement due to patent infringement I think.

I think there are some Javascript hacks to get around this - Google "Eolas patent" for more info.  The quick version: Eolas has a patent on something regarding browser plugins (fairly or not, the USPTO decided in Eolas' favor), Microsoft got sued, and had to add the "Click to activate" thing to try to dance around the patent (though this wasn't court ordered, it was likely an attempt to show a good faith effort not to continue infringing or something like that).  Firefox, so far, doesn't really seem to care if it infringes upon the patent or not, probably because the devs don't think it's likely someone is going to come to them to score easy money - after all, the first rule of patent trolling is to go after the rich companies first.

Microsoft apparently finally settled with Eolas, so maybe in the future this will go away.  I'm not sure, though.