You could argue that they should be supporting accounts for kids under 13 with parental permission (like Yahoo does, for instance), but that's not the same thing.
Edit: and to preempt some class of responses: yes, as has been covered on HN before, COPPA has a clause that says not to warn users that setting their age to under 13 will prevent them from accessing content, and that you can't (generally) let them go back and change it later.
If Facebook/Twitter bans you, you don't lose access to Gmail. It makes no sense.
It's not "easier" to do that, it's just what you have to do since it's so difficult to make any social / messaging service COPA-compliant.
I'm not sure that COPPA applies to GMail. Certainly, Yahoo mail doesn't stop me from entering a birth-date under the age of 13 when signing up for it.
That's different from how they would handle it if they had an existing account on Yahoo Mail and found out that the owner was less than 13 - I suspect they would have to turn it off as well, because they won't let you create a non-COPPA account once you've admitted you're under 13.