The part that got me is "must patch any critical bugs in the game within a month of release". I'd much prefer you know, "game must release without any critical bugs". I still remember the days before patching and how companies had to pretty much get it perfect the first time. Seems like a clause for the current lazy "we can always fix it later" generation of developers who want to release and move onto their next project ASAP. I can't help but wonder what would happen if there was an actual "no major bugs or you don't get paid" clause, but I kind of suspect it would mean a lot more serious playtesting and QA and better games, since I really don't think it's an impossible goal to hit given how many titles have been released in the past before patching that worked fine without "critical bugs".
That said my basic thought on the conversions is that they want to maintain as tight a control of the property as possible. If someone else does a conversion. paid or free, there are questions about the ownership of what's in that conversion, which in many cases will add things like new graphics, weapons, elements... etc... to a game. Even if someone doing a conversion doesn't push things legally, being perceived to rip off a mod or conversion can hurt a company's reputation, especially if they don't acknowlege it. There has been a LOT said about these kinds of things over the years, and I'd imagine Activision just doesn't want to open that can of worms.
Let's say someone decides to do a total conversion of the game, similar to what the guys at SCP have done. They add a new gameplay element with a monster that moves quickly and kills you if your too close if your not looking at it constantly (and while your looking at it, it's pretty much harmless), SCP pretty much did that. Now all of a sudden there are questions if the actual developer actually decides to use that without acknowlegement for an actual game, and make money off of what those guys did. This is no big deal when it's fan conversions (which they don't seem to be worried about), but if a company like Valve or Epic gets pissed about something from one of their conversions, right or wrong they could keep the property tied up for years. Having a leg to stand on legally isn't even the most important thing, I mean consider the article just put up here on The Escapist about Epic Vs. Silicon Knights, where SK pretty much got pwned not because of their case but because of an expert they relied on getting deep sixed right off the bat for what are to us some pretty vague reasons, and which might have had more to do with technicalities than the case. I don't presume to know anything there, but if you were Activision, would you want to fight Epic's lawyers? Sure Activision can hire their own bastards, but at that level it's a coin toss since they would both be acting with deep pockets.
Just my theories
Oh and on the easter eggs, all I have to say is "why bother to have them then?". An easter egg can't be awesome and spontaneous if it has to undergo corperate approval. The best easter eggs are the ones aren't planned ahead, and just things the coders decide to do for Lulz when it tickles their fancy at the moment.