The principle of not reinventing the wheel has nothing to do with interview questions. Sure, if you're actually working on solving an actual problem at your job, then most of the time you'll be better off using a standard library function instead of rolling your own.
In the context of the job interview, however, it doesn't matter if the solution to the problem is well-known or whether it can be found in a standard library. The problem of the interviewer is not to find out how to compare strings, it's determining if the candidate will be able to write proper code if hired. And the string-compare question might be a good starting point for evaluating the candidate's level. In any case, the fact that a solution is in a standard library is irrelevant.
If I was the interviewer I'd create a new data structure Foo and then ask the candidate to implement Foo.compare(). The question remains essentially the same, so I wonder what the candidate would reply.
