For all I know, McCain may well have called around to Republicans. But as John said, that doesn't explain why the Democratic co-sponsors didn't sign onto the discharge petition.
If the bill is not going to pass the Senate anyway (which it would not), I see nothing wrong with making the Dems push it thru with their own votes. Dems should not get credit for pushing thru an immigration bill that was passed with GOP votes. Make them work for their votes.