only buy these certificates if they are relatively cheap, ie. $3. if it's $10 or so, i would stay away from it because i've had a number of restaurants turn these gift certificates down.
these are not actually gift certificates in the traditional sense in which merchants are obligated to accept them. READ THE FINE PRINTS!!! most of these GC have a minimum requirement of at least a party of 2 or more, some require a party of 4+. it's only good for dine in.
the restaurants accepting these GCs are basically giving you $25 or whatever the face value of the GC you are using for free because they are not compensated monetarily by restaurant.com. the restaurants are getting advertisement from these gift certificates.
some restaurant owners have decided that giving away free food is not the most economical form of advertisement. if your GC is turned down when you try to get some cheap eats, you can email restaurant.com w/ the reference # on the GC and they will refund whatever you paid for it.
if the GC is still valid (within the expiration date) and have never been redeemed, it's probably better to just leave the GC plus whatever the difference in cash plus tips, then dig out. that way, they would have to bitch to restaurant.com about it if they no longer accept these GCs.
they can't call the cops on you because legally, you did pay for your meal. If a restaurant went into an agreement to accept a certain form of payment, they should be obligated to do so until the agreement expires, which should be the expiration date on the GC and not any sooner.