Roster churning is unethical. Period.
However, if he feels he has the roster space to carry X amount of Quarterbacks and does so because he feels it will put him in an advantageous position for trades or whatnot with owners that are thin at that position then that's strategy, and it's his business.
I just traded a RB who would have never seen my lineup (L. Washington, who I scooped off the WW) for some much needed WR help (Driver) in one league because (according to some of the other owners in that league) I "hoard" Running Backs and lo and behold someone needed one. Does that make me unethical or smart?
Running Backs are GOLD Jerry....GOLD!
And maybe, just maybe, in your league, that owner feels the same way about QBs.