Wow, great points by both Enzo and Turnbull. Mike did lure Carlo into his trap there when he dismissed Tom.

My whole take on this was that Michael and Vito had decided to take the family to a whole new level, and part of that was severing old ties to consolodate Mike's power, show by example Mike was "the one" and there was no going around him, isolate the decision making (at the time, there was still a rat), and the need to have a muscle guy to the advising (Vito) instead of having a legal guy (Tom).

And I think that if anything did go wrong with what they were trying to do, Vito needed someone to hold things together, and that would have to be Tom. He did say that it was important that Tom "not be a part of what was going to happen."

Of course, this does not explain how badly Mike treated Tom, even well into Part II.