In a bid to crack down on nefarious pirates, Microsoft is issuing permanent Xbox Live bans to folk caught playing illegitimate copies of Halo 4 early.
Reddit has a post containing the actual ban warning, which contains the usual spiel along with a note that non-XBL users may be getting the email due to their activity on Zune Social. I only mention that because the idea of the Zune makes me laugh on the inside.
Anyway, it should go without saying that if you simply must play Halo 4 early and free, maybe you ought not to jump online with it. Consider it.