There are some consensus rules against invalid moves, which are actually checked and verified.  Invalid paths (or at least some parts of them, running through obstacles) are not part of that, though.  This was already so in the earliest versions, and I've simply never changed it.

I don't know if it was an oversight or how it came to be.  One reason might have been that it is not fully trivial to check an array of waypoints, since you have to replay them and simulate the whole path.  Maybe Mikhail thought it too complicated or expensive to do that for every tx for the benefits it has, especially since doing that for unconfirmed transactions could have created DoS vectors.  But that's just speculation.