The main reason mass-market paperbacks are bound so poorly is that there are a tiny (and diminishing) number of companies that print and bind mass-market paperbacks, and a great deal of demand for their services. In this sellers’ market, publishers count themselves lucky just to get time on the presses. I believe (this is in reference to your commenter) that, for their mass-market titles, Baen uses the same manufacturers that Tor does for theirs, and that the quality they get for a particular run is somewhat dependent on luck. Older houses like Bantam and Ballantine can sometimes get better work, because their deals are more longstanding and their volume is higher, but they wind up with some badly-bound books as well.
Understand that I’m talking entirely about mass-market, rack-size paperbacks, not “trade paperbacks” or other books.
There you have it.