You’re thinking too much like a programmer. It’s a single rule for a single domain that accounts for the majority of users.
The right thing to do is ask for a password, but absent that, accounting for gmail addresses seems worthwhile. It would save them getting a thread like this on hn.
* Gmail: Dots are cool.
* Hotmail: No capes! I mean dots. No dots!
* Multiplied by 8 hundred gazillion domains...