The players union and the league will negotiate terms for a new multi-year contract. But there has never seemed to have been a discussion of trades made during the regular season.
Some players have no trade clauses, i.e., the player must approve, in their individual contracts. And there are general rules about players with a certain number of years in the league and/or with a team having similar rights to approve trades.
But none of this forbids trading players during the season. Why not?
Do the players think that it is an essential part of how a league must operate? Are they just lazy in their thinking?
On the other side, why do the owners think that it benefits them? Is there actual evidence that undermining fan loyalty to particular players is offset by some future improvement in team competitiveness? Does continuity and player loyalty impact performance?
And, of course, as has been emphasized here many times over the years, trades during the season undermine the basic integrity of the game far more than players using performance enhancing drugs (PED).
Oh, and as long as they're fixing fundamental problems, have the same number of players on the roster for the entire season, i.e., finally end the absurdity of expanding the rosters in September to accommodate meaningless call ups from the minor leagues.