Re-endering the track is not illegal, but re-endering it through the side of another car is both causing an avoidable collision, and failing to reenter the track in a safe manner – he was punished for the latter.
I would have thought he should have been penalized for causing an avoidable collision.
What was Senna penalized for?
He was punished for Deliberate Crowding of another car beyond the limits of the track. Personally, I have no idea why.
Neither do I. What was different then between Senna crowding the Sauber beyond the limits of the track, Hamilton crowding Maldonado on the exit of the right hander or Rosberg crowding Hamilton in Bahrain? Why is the first instance considered "crowding" and the other two not? Here lies the problem in the definitions of the rules in my opinion: "deliberate crowding beyond the limits of the track" does not consider whether there is a wall or not on the outside, so this should be irrelevant.
Just for reference re Igor's penalty – the last person to fail to rejoin the track in a safe manner was Lewis Hamilton, and his penalty was a drive through penalty, applied after the race, and hence 20 seconds added to his time. Of course, when Hamilton did it, he didn't hit anyone, and didn't plant them in the wall.
When was that? Last time I remember was Hamilton overtaking Rosberg in Bahrain by going off track and re-entering as if nothing happened. He did not get a penalty for that. Rosberg was investigated and found not guilty.