Here's how I understand the situation: private-equity firms got banks to back their acquisitions. The banks agreed to raise a certain amount of money, generally by selling bonds. Now the banks aren't finding it quite as easy to sell the bonds to investors. So who is interested in buying the bonds at a dicounted rate? Private-equity firms, who can raise additional money from their investors! (Financial Times article)
Now, it's not at all clear to me that we should be blaming private-equity firms for the credit crunch. But even if they were to blame, I don't see them taking advantage of the bank's situation as unethical in any way. If I may be excused a poker analogy, a bad preflop call doesn't mean you can't try to win the pot postflop and "benefit" from your mistake. (In this case, it seems to me that the "mistake" is in fact on the bank's part--- they thought the credit market would back up the loans they agreed to.)
If a private-equity firm knew that its partner bank would suffer and set up the situation in such a way as to profit, that might cross the line. But either way the responsibility lies with the bank not to get itself into bad deals, not for the private-equity firm to show "restraint" (or whatever the opposite of "overreaching" is.) Nor should the private-equity firm turn down a profitable investment just because they (might have) had a hand in the situation which provided it.