This makes sense. Currently there is no room for him on the roster. It also means the Lee trade will not be completed today, as Pressey could have otherwise been included in it. Babb will now be sent with Wallace, as the C's continue to try to use their excess cap space in the meantime.
However, for Pressey fans, this is a good thing. Assuming he clears waivers, there is nothing preventing the Celtics from bringing him back (if, say, they traded another PG in a larger deal). If he were included in the Lee trade, they couldn't reacquire him until next year.