All part of her cover though... She's trying to save her ass, and doing the "diplomatic protest" thing to basically say "nah nah, you can't touch me". The Imperial senate itself seems to be an issue for Palps/Vader until the DS's construction is completed. WIth the "ultimate weapon" in action now, they're planning on using THAT to keep everyone in check. It seems to me that there's a good many years between E3 and E4 where there's political turmoil and in-fighting. I guess the declaration of a new Empire didn't mean the declaration of a totalitarian dictatorship by any means, and that many planets were threatening to reopen a galactic-wide war if they didn't have their fair say in a Senate.
Palpatine/Vader must go through a lot of **** for a long time I'm guessing, and then in ANH the DS is built and Palpatine dissolves the Imperial Senate based on the news that it's ready to roll. Tarkin, who seems to be quite high-ranking, takes it for a test-drive and uses it to A) make a statement to the galaxy as a whole by blowing up a core world in Alderaan, and then B) also is using it in an attempt to hand the Rebels their collective asses, and hopefully quell any other upstarts.
I've not played the game through yet but that still jives to me... They know Leia's "involved" they just are having a harder time proving she's active. Catching her stealing secret documents though and giving them away? That's a good way to put the Rebel logo to her publicly and out her entire planet, perhaps justifying a full-scale invasion of Alderaan at the time? What better reason to declare martial law than if that planet's leader is a clear traitor to the established government? Destroying it is just an afterthought by Tarkin to get his point across and deal with a "problem" all at the same time.