With the limited information I have about how Rewards Network works, I'd have to not make a judgment. But I can see how some restaurants might opt out - locally (near SMF) we have lots of AA and UA flights, some DL, few NW and CO flights. If there's a cost associated, it may be a restaurateur may decide to eschew certain airlines due to the (even if merely perceived) predicted lack of customers.
Can anyone illuminate what a restaurant pays for Rewards Network membership and benefits?