It is a problem for freelancers - people always ask for free solutions to real problems and various ways. Candidate employees are in the same situation, but during the worst possible time - the interview.
As you said, you want to hire people for whom solving your problems is easy. Keep in mind that your problems are not unique and probably already have a solution, so you really need someone that.....woops...almost gave it away ;)
Though I doubt this is a problem in interviews. But asking for a solution even before the interview will skew your distribution of appliciants, like you point out.
Not sure if this is a bad thing --- you want to hire people for whom solving your problems is easy.