Uneven balance of the muscles = turns the eye one way or another, depending which way more force is exerted. Makes sense, no?
Both tight? That would be a neat trick, because they are antagonists. When one contracts, the other relaxes. If they were both contracting at the same time, you couldn't move your eye.

Let's suspend that fact for a minute and assume that the muscles are actually constantly tensed and causing the eye to be compressed and deformed.
First of all, don't you think the muscles would become fatigued quite quickly from the constant contraction? Try looking to your extreme left or right. See how long you can keep that up without it starting to ache.
Secondly, the inner lining of the eye, which is responsible for your vision, a.k.a. the retina, is not so resistant to deformation. Your vision would be distorted because the cells perceiving the image would be deformed. The bigger problem is that this layer does not even accept any deformation - it just rips. This is why boxers get retinal detachments when they get punched in the eye and it gets compressed, or even sometimes from transmitted trauma from a punch to the skull.

What % of people get any improvement from wearing glasses? About the same % for which glasses are prescribed as a permanent cure from their refractive error.
What % of people have any sustained improvement from pushing on their eyes? Probably about the same %s.
There aren't any controlled longitudinal studies done on it; would be a nice place to start for doctors who claim this works.

You suggest that glasses are bad. Why? Do they make the muscles tighter? How?