I don't think the policy has been changed but the "request an invitation" feature from e-Rewards website hasn't been on until recently.

You should see "Request an invitation" if you go to their website. Currently there are a bunch of sponsors you can select (Blockbuster, Border, Hilton, various airlines..).

I would think it should be perfectly fine for those who don't want to be restricted to Skymiles in terms of airmiles redemption (meaning they had used Delta's invites to join e-Rewards originally), they should be allowed to request another invites from a different sponsor AFTER cancelling their current accounts.

From e-Rewards Member Services:
11. How do I cancel my membership?

To cancel your membership, log in at Member Login. Click on the "My Account" tab and select "Cancel Membership". Complete and submit the online form to cancel your membership.

IMPORTANT: Once you have canceled your membership, all e-Rewards Currency you have earned will be forfeited, and your account CANNOT be reactivated at a later date. In the event you wish to participate at a later time, you will be required to request a new invitation.
(Bolding Mine)

No idea how long it will take to receive the new invites though.

BTW, if you are up for this cancel-then-rejoin route, be careful to which sponsor you are going to choose. If history is any indication (e-Miles sponsored--> e-Miles only redemption; Delta-Sponsored--> Skymiles only redemption), airliner-sponsored accounts could be less flexible when it comes to airmile redemption. So choose wisely and live happily!