According to Legends (C-canon) game Star Wars: The Force Unleashed, the Rebellion's start was as follows:
Galen Marek, pretending to be a Jedi, convinced Senators Bail Organa, Mon Mothma, and Garm Bel Iblis, and General Kota, to start a formal open Rebellion Alliance (Corellian Treaty in ~2 BBY).
In reality, Marek was Darth Vader's secret apprentice, and the whole thing was part of a plan to lure all the potential rebels into the open, under the pretense of creating an alliance to destroy the Emperor (and in reality, to flush them out and crush them).
The plan initially succeeded - at the signing, Vader and stormtroopers arrested Bail Organa, Mon Mothma, and Garm Bel Iblis, and took them to Death Star I to meet the Emperor and their doom.
This was disrupted when Galen Marek switched to the Light side, and at the cost of his own life bought the escape of the leaders of the soon-to-start Rebellion.
Question is: that was 2 years before events of SW:ANH, and for those 2 years Vader and Emperor knew that Bail Organa was a rebellious traitor.
Why didn't Palpatine crush (either arrest, or assassinate) Bail Organa (or the others, for that matter - but with Organa, he knew where to find him, on Alderaan or in the Senate) in between 2BBY and Alderaan's destruction during ANH?
Answers from any pre-Disney level canon are fully acceptable.