Talked to the developer responsible here; the original plan was to integrate the Area 51 opt-out process into the new system used by the other sites... But, this hasn't happened yet and may not happen for a while.
So the change has been reverted and should now be live. Future emails from Area 51 will again contain a quick "unsubscribe" link that simply turns off email notifications for proposals you've followed or committed to.
As I said in the comments above, there should be no legal ramifications here; we're not spamming people with emails for proposals, you'll only get 'em if you've gone out of your way to follow or commit to a proposal (at which point, further communication regarding the progress of the proposed site should be expected and would fall outside of the marketing definition used by CAN-SPAM). Nevertheless, it is an expected convenience, and given the option still exists to opt-out of these on-site...
...it seems appropriate to also provide it in the actual emails.