This is a good point that may elucidate how such things are tested: someone conjures up the 'contract' / terms and conditions, then what? Is there a suite of 'unit tests' for the conditions, then a QA department? I imagine there is (or was) an economic analysis of this that leads to the decision to apply human resources elsewhere within the company. The 'loss' from a few exploiters must be probabilistically low and does yield publicity / free advertising (right?).
Too bad there's not a fuzzer for English.
It's called a lawyer.
And the tricky bit for something like this is that the terms and conditions have to match the programming, otherwise someone might exploit the programming, even though you wrote bulletproof t&c.
The general way out for the airlines is that the terms give them the right to unilaterally change the terms at any time, or terminate membership in a program at any time.
I think adding "Limit: One rental per day" would have solved this problem.