I'm totally willing to believe this is Sony's fault - note that I did say "stupid reason".
Let's get someone from Harmonix in this thread to say it, though. Sony's method of deciding which games get platinum trophies (usually all retail games, but also some downloadable games), which games get DLC trophies (you know, the trophies with a + on them) and which downloadable full games only get a limited set of trophies seems to be completely schizophrenic. It seems to be related to price of the game, but only vaguely.
I understand why Blitz might not be allowed a platinum - hey, it's only a $15 game, I get it, Sony doesn't want their "trophy economy" ruined or whatever. Still, it's hard to believe that Blitz wouldn't be allowed to add those DLC trophies - the Rock Band series has got to be the biggest pusher of DLC that exists on PSN! Did Harmonix even attempt to "apply for" (whatever the process is) DLC trophies for Blitz?