I don't think there necessarily has to be a problem with what you're describing in theory, but something sounds fishy.
As a long-standing keeper league commish, I don't tend to veto a trade unless:
- I think it was made unethically i.e. a team made the trade for some reason other than to better their own cause
- It is so horrifically unbalanced that I fear it really jeopardizes the integrity of the league. I think I've only done that once ever
In this case the thing that seems iffy isn't that a team is trading a player they aren't going to protect for a 7th round pick. It is that it suggests that all the other teams in the league wouldn't give up a 6th round pick, assuming that they could protect 8 players better than Fielder and still get a better player than him in the 6th round of the draft. That seems terribly hard to believe. In a 2 team league, the two teams would protect 16 players, then draft 12 more before getting to the 7th round. So it is pretty easy to make the case this trade is nonsensical in a 2 team league. If you were even an 8 team league, for example, the same scenario makes no sense. A disinterested third party decided that after 64 players were protected, and another 40 players were drafted in the first 5 rounds, they still would rather have their 6th round pick than Fielder. That's just bananas. When a trade gets protested in my league, the first thing I do is ask both teams to explain their rationale in making the trade. I'd be fascinated to hear why the guy trading away Prince thought he couldn't do better than this. And any time I'm resigned to allowing an unbalanced trade, I remind teams to shop around and make sure they're doing the best they could. This almost can't be true in this case.
The format of your league makes this relatively simple math. A few years after starting to do keepers, my league quickly decided that a straight "you protect X guys" rule was overly simplistic. Once you start associating costs with protecting guys, be it forfeited draft picks, increasing salaries, etc. etc., then it gets really weird. It is easy enough to say that of course Prince Fielder is one of the best 100+ players in the game, and thus worth more than a 7th round pick. Suppose you get to protect 20 years of service and Prince Fielder costs 5 of them. Now how valuable is he? What if he costs 7? etc. etc.