Probably because of the (very small) chance that Copilot could regurgitate something secret or embarrassing.
Which is not necessarily hypocritical. The amount of copying needed for something to be copyright infringement is not high… but it's still significantly higher than the amount needed to leak information. For that, just a few words will do, e.g.
// For Windows 12
or
// Fuck [company name]
or
long secret_key[2] = {0x1234567812345678, 0x8765432187654321};
