Charging to recoup the cost of the infrastructure = ok.
Charging more to force competitors out or to build their own, wholly redundant, towers = illegal.
It's essentially a matter of declaring that the towers themselves, and the infrastructure that connect and power them, are not to be a profit maker for anyone. Can be made a condition during spectrum allocation, for example.
There's still plenty of other areas for providers to distinguish themselves.
And there's still an incentive to increase coverage. The only coverage increase incentive lost is the incentive of becoming a local monopoly. Good riddance, I say.
They're already required to make sure the tower doesn't fall in someone's head. That cuts into profit. But they still do it.