There's a predefined window for when a device can be certified for release with a particular Android OS version. I don't know what the current window is, but devices that are shipping now (Q1) likely got certified some time last year in Q3 or Q4, well within the window for N.

It's significantly harder to enforce things like update requirements after a phone ships - for one thing, it's not entirely up to the OEM in many cases, it also depends on getting carriers to certify the upgrade.