IANAL, but in general the penalty for breaking the terms of a contract, in the absence of an explicit damages clause, are whatever business loss the other party suffers as a result.
Any binding agreement between two parties is a contract. There is consideration on both sides: IFTTT is providing access to their dev kit and service, the recipient is implementing an API and providing access to their data. Both sides are, in principle, getting something from the agreement (of course, it's very lopsided and unfavorable to the recipient). Again, IANAL.
