Exactly. If none of these are happening, "collusion, team dumping or cheating", then you should not veto.
In a keeper league especially, managers should be able to take chances/gambles in trades. At the beginning of the year, someone tradeing Biedrins for Lopez would have been crazy, but now look. Why take away the ability for someone to buy low on someone they expect to blow up??
I say, don't veto 99% of the time since you don't know what that player wants.... heck... in the NBA Memphis made some questionable moves (the gasols) but now they look as if they might pan out..... you never know.