IN RL, teams make WW claims to prevent players from clearing waivers and being trade-eligible, and claim players in the Rule V draft to force other teams to keep said players on their rosters. It's not mean-spirited, poor sportsmanship, and petty; it's playing by the rules. If an owner automatically vetoes a trade that he perceives will improve a competitior, go for it - if WIS doesn't want vetoes, they'd disable the feature. I don't do it, but I'll admit I look at trades involving teams in my division a tad more closely than I do other trades.
Where I think there may be a gray area is in the Fair Play guidelines:
"IMPEDING OTHER OWNERS
"Certain transactions made solely to impede other owners are not allowed. For example, protesting a trade merely to help someone else is strictly prohibited."
If you want to interpret this loosely, you could make the argument that vetoing a trade simply to keep an opponent from improving is impeding them (but then again, so is drafting '08 Joss...). I don't interpret it that way, and seeing as this wording has existed as long as I've been on the site if it was meant to be interpreted as suggested I think it would have been applied by now to un-veto trades.
Also, I can easily see why redsox1966 thought he was being insulted:
Person A: "I perform such actions for such reasons."
Person B: "I find performance of such actions for said reasons is mean-spirited, poor sportsmanship, and petty."
Person A: "Are you calling me mean-spirited etc.?"
Person B: "No, I'm saying the actions you say you do for the reasons you say you do them are mean-spirited, etc."
Parsing a political spin doctor would love ...