It isn't uncommon for threads to be moved from the general forums to "Politics&Religion", if containing controversial subjects, effectively locking out some of the previous respondents.
As far as locking threads. I do try to give thoughtful replies, but have seen several threads I wanted to reply to that seemed to be abruptly locked. I suppose I can see the benefit of locking them down if they get too personal, or drift too far from the OP's subject, especially for certain topics that cut close to home.
Perhaps in some cases a firm warning to return to topic would be better than locking. Or, first warn, then lock.