Vetos are at the discretion of those voting. I haven't seen any fantasy sites that suggest that the only grounds for a veto is collusion. This trade is really bad, and I'd probably veto, but I'd have to know a little more about roster context. This year in my H2H basketball league, some bad trades were made at the end of the year between playoff teams, and teams not within a sniff of the postseason. I will allow trades by teams that are very active and clearly trying to win more liberally than I will by teams that don't log in and set lineups for a month and then all of a sudden trade their first rounder for a 6th rounder and the WW flavor of the month. You have to look at intentions first. Then yeah, Pujols for Finley and stuff like that should also be vetoed right out of the water, IMHO.
Yahoo words it "If four or more managers feel that this trade is *unfair* it will be vetoed." Unfair can mean more than just collusion.