IANAL...but fuck that. I assume your previous communication about the 'unofficial version' was in writing, at least by email. So legally, they don't have a leg to stand on because they've already folded and accepted your right to supplement their service with your little app. I(f they had strenuously objected straight out of thegate that would be a different situation, but they've already committed to a course of action and now they're backtracking. You can show you dealt with them in good faith (I presume) so I think you're quite safe with 'thanks, but no thanks'.