I don't think they need to worry too much about abuse.

I think part of the value prop here from mojofiends perspective is that they are providing coffeeshops potential new customers. The coffeeshops that are offering them a $5 coffee don't care if the incoming customer has spent only $1.40 on coffee before today much the same as they don't care whether they bought a new tv yesterday. They just want access to potential new customers.

Groupon has proved how much businesses want potential new customers, I think this is an interesting twist and will perhaps generate better leads for coffeeshops than a groupon would.