This is a marketing observation rather than compliance:
Christmas clubs can be a good way to get new business. Some people put their Christmas club at a separate bank from their other accounts precisely because they want to make it less convenient to pull the money out early. But if they have good experiences with you, then they start to think maybe they would like to have other accounts with you, after all.
Now, you could view it two ways: Either requiring them to have a checking account could nudge them a little farther toward doing more of their banking with you, OR it could make them think "If I can't open just the Christmas club like I want, then I may as well have it at my own bank."
Another issue is that plenty of people are a low risk in a Christmas club but not people you want to give a checking account.
Opinions expressed do not necessarily reflect those of my employer or of my cats.