Loss of employment may ultimately come for a GM who makes poor decisions but a.) that doesn't stop the poor decisions from happening prior to the unemployment and b.) it doesn't assume the new GM will be better (just ask Seattle fans).
When the Yankees acquired Bobby Abreu from the Phillies, there wasn't a soul in the baseball world who thought Philly got anything close to fair value for Abreu. In fact, the four players they got for Abreu (and Cory Lidle!) have combined for 67 major league games played. It was an absolute dumpster fire of a trade. And yes, that trade affected the other 28 teams in the league. The Yankees got MUCH better, the other AL teams were adversely affected by the Yankees' improvement, the NL missed out on a chance to acquire Abreu, etc. etc. It was an awful, awful, AWFUL, trade. And there have been countless similar traded before and since that one. And not a single one was struck down by the league. Because it was up to the two teams involved in the trade discussions to determine if they were getting fair value on their end. Nobody said "wait, Philly is making a really ****** trade here!" and stepped in. Nobody made an "attempt to stop the Yankees from building up their franchise." The two teams came to an agreement and that was that.
If the stud CF your trading for a handful of spare parts is going to a team that is already stacked, while YOU may think that's best for your organization, can't you see that OTHERS may not think that it's best for the world?
What if your bad trade cripples your team so badly that you decide to up and leave at the end of the season? Now the world has one team that's even more stacked that it originally was, and a second team that's been crippled in need of a new owner. Don't you see that the other 30 owners might see that as harming the world with competitive imbalance?
I'll flip it around: what if the only team that I find to be a reasonable trade partner is a stacked team? Am I forbidden from dealing with that team simply because you don't want that team to get better? Should I be forced to sit on my pieces because I can't find any other trade partner I want to deal with?
As for someone leaving a team after a crippling move, that's what vetting owners is for. It's pretty easy to tell when you look at a HBD owner what kind of track record they have and if there's any risk of them doing something like that. I'll grant you that if you were to veto a trade like that made by an owner with a shady track record it would be more reasonable. But if it were someone like Mike who has a long history playing these games, who everyone has a pretty good feel for, I'd let it go without a second thought because I don't see him as a risk of jumping ship because one thing went wrong (and I'd let it go because he has such a long history that, even if I see a trade as terrible for him, I'm guessing he sees something in the player evaluations that I don't see).