Sure, but regular quantum mechanics does the same thing. It's under-constrained and allows for too many possibilities. So you assume that the observations wrought by Newtonian physics must be valid and use that to constrain the theory down to something that can be useful.

The difference is that the string model doesn't (yet) have any observations which we could use to constrain it in an analogous manner. But in studying it, one would hope to find areas of the theory that indicate where those observations could be made.

The point being, it is not so simple to just look at string theory and say you can a priory determine it is flawed or wrongly motivated. That same kind of reasoning would likely have prevented Planck & Einstein from starting quantum theory.