Employers DO have the right to require sobriety and to enforce such any way they see fit. They are not the government, they can hire you or dismiss you for any reasons whatsoever or for no reason at all (assuming a state with at will employment). Finally if they request or require sobriety and that doesn't jive with you you can leave. There is nothing keeping you at that job. You have agency and if that companies philosophies differ what what you want to help support you can change to a job who's beliefs are more inline with your own.
Corporations in America exist in a free market, a capitalist system. You are free to make choices and so is the company. If suddenly drugs became so commonplace that a company was having difficulty filling its positions with abstinent workers you better believe those policies will change very quickly. Similarly if consumers are demonstrating that they expect a company to be associated with 0 tolerance with drugs, and doing so by purchasing only from those businesses you can ensure that companies will continue to drug test.
The point of all this, finally, is that companies will do whatever they think is in the corporations best interest. Consumers and employees essentially dictate what beliefs a company will endow and it's fallacious to place the blame on the company entirely, if in any part at all.