I compiled all the avatar awards I'm aware of into a guide.  Take a look and tell me what you think.

Many of the games with avatar awards are XBLA games.  My belief is that since the prices of XBLA games has risen dramatically since the service launched and the amount of Gamerscore allotted hasn't, awards are a way for the game maker to provide an additional incentive to play.  At first XBLA games were usually $5, now they are $10-$20 with $15 pretty much being standard for good ones.  $15 for 200 (potential) gamerscore is like charging $75 for a full retail title.  
 
I'm not saying points are the only reason for playing, but I'm sure I'm not the only one who looks at the achievements available when they are on the fence about getting a game.  
 
I also suspect (and this is dangerously close to conspiracy theory territory) that MS is pushing companies to include them (or providing them to games).  Of the 4 games in Block Party, 3 have awards, and there is an award for playing the demoes.  And the only game without them isn't really a game.  It is just as likely that developers are just now getting into it and it's all a coincidence, but I have a suspicious mind. 

ODST has avatar awards? i completed two of those achievements and i never got the t-shirt! :( 

Yes, I need to clarify that, thanks.  All the awards are technically from Waypoint.  And by my own rules the Guilty Spark shouldn't be included since it's not obtained via a game action.  But since it has all the other ones I put it in.