It comes to a point where we just need to agree to disagree. I believe you should let owners manage their teams and vetos should only be used if there is collusion OR if the trade is so lopsided it effects the competitive balance of the league. In this particular trade, I would need to hear the owner's reasoning. If he doesn't provide a sound explanation or I get the sense that he just doesn't care about his team or this year, then I veto.
For everyone that says "Do not veto unless collusion. Period.", would you approve a trade of Peyton Manning for Roy Helu? Here is the background: The Manning owner believes that Peyton will not play much the week of the FF championship due to the broncos locking up the #1 seed. He didn't spend much time trying to get the most for Manning, and likes Roy Helu's potential from here on out. The owner getting Manning has a sick team, except for QB. With Manning, he now has the best team by far in this league. I'm sorry, but this is bull$hit.
Let me put it another way - people have lives outside of managing their fantasy team - kids, wives, jobs, etc. Some owners just don't put that much time into their teams. You can't compare them to an actual NFL GM, who gets paid millions of dollars to manage an NFL team. If an owner is not in collusion, but just makes a horrific trade because they don't have the time or energy or smarts or whatever, I believe you veto the trade so the rest of the league doesn't suffer (competitive balance).
Please come and bash, but I hope I'm getting my point across.
Our positions are not that far apart, really. Mistake #1 on the part of that OP commissioner was not contacting the owner who traded away Calvin and Cruz and getting an understanding of his reasoning behind the trade. At least he made no mention of having such a discussion. Importantly, i believe that this is something that should always be done by phone , NOT by text. You want to have a conversation where you can actually hear the answer. IMO, you have no grounds to veto the trade unless you discuss the trade with the owners involved and hear it directly from them.
In all the leagues I have been commissioner, I've never had a case of collusion....ever! But I have vetoed one trade and reversed another after I approved it (Interesting case - the very next day, the idiot dropped the only fantasy relevant player tat he traded for. There was no 'foul play' involved. He was just invited into the league (unfortunately by his trade partner, and was in completely over his head. I'll give him credit - after I talked to him he messaged an apology to the league and voluntarily forfeited his $150 league fee and deposit ).
But my standards for vetoing a trade are more restrictive than yours. The only reason other than probable collusion where I will veto a trade is where, in my opinion, the league would totally disintegrate if i would allow that trade to stand. I have no problem dealing with pissed off owners, but if I truly felt that the league would fold because of that trade, then it is my responsibility to protect the league. Unlike your Peyton Manning for Helu example, I don't believe that the above trade would rise to that standard.
Regarding someone's idea of not inviting the guy back next year, I come from a background of playing a pretty decent level of poker, and my question is this: Why would you not want to keep some dead money in the league, even if you didn't happen to be the beneficiary of a lopsided trade? I've also seen many a fantasy owner (including myself many years ago) who made some TERRIBLE trade decisions when they joined their first league, but then become quite skilled, learning by their mistakes. Come to think of it, I still make my share of 'bad' trades where the other guy benefits much more than i did. To again use the poker analogy, when I played online poker for money, I'd watch a table for a while before 'I took a seat'. If everyone was a shark, I would look for a different table. Why would i want to play for money where everyone was as good or better than I was?
Back to fantasy football - The cool thing about it is that player valuation is both fluid and subjective.