According to http://archive.nyu.edu/bitstream/2451/15020/2/Infosec+BOOK_T... “experimental studies have shown that the bulk of victim credentials are collected within 24 hours of mailing the bait messages.”

Once a phishing form is “in the wild,” every minute counts.

The burden is on the service (your site) to prevent or quickly act to rectify a situation, but if your provider determines that it must intervene, then it is well within it's right to.