They are supposed to simulate fan voting.
If you click on All Star Weekend on your Standings Page, and then Select "Season 3" at the upper right, then click on "Voting Results" towards the bottom, you will see the voting results from last season.
In my league there was a very dominant SG (possibly a NT player) who led the voting with 21,145 to 5804, 2343, and 1378. I believe it is based on actual game performances, rather than the skill ratings, and so if you get a player just before the All Star game, he probably won't be selected.