I think we should have a sticky (and maybe locked?) thread in this forum for "When to Veto a Trade". When you look at the number of people that ask these questions, how many more don't ask? Maybe we could be preventing Commish injustices around the world if we had a thread dedicated to Trade Vetoes.
Collusion. If you can't prove it is collusion, or unless all the other league members agree it appears to be collusion - DON'T VETO THE TRADE. As a Commish, it is not your job to manage other owner's teams. Your ONLY job as Commish is to manage which owners you have in your league. If you have owners you can't trust or who always make awful trades, replace them. Otherwise, let owners manage their teams and you manage yours. Stay out of their business.
A trade like this one, I'm not even sure who is getting the better deal. Team A is getting a currently underachieving one year wonder and a handcuff (albeit a good one) - Team B (who I assume is dealing with injury and needs bodies) is getting three bonefide starters that I'm guessing will upgrade his lineup at 3 different positions. Why would you even consider punishing either or both of these owners by vetoing such a trade? I would venture to guess Team B prior to this trade could not even field a full healthy lineup.
Questions like these over and over every season really get me going. Not so much upset, but I just don't understand what the logic is to question a trade like that which is clearly not collusion, but also not remotely lopsided. The only eyes that appears unfair to is the other one or two good teams in the league who now see two other teams get better. And to veto a trade because now your team won't be the best anymore? That's just poor sportsmanship.
(Disclaimer: I'm not as upset about this as I'm sure it sounds, but I just watched my Brewers butcher a playoff series, so I'm easy to set off right now...)