It's kind of like how if you owe the bank $1000, you have a problem, but if you owe a bank $100M, they have a problem. You just can't reasonably ignore a huge portion of the class as a professor without a serious amount of documentation, and proof that you've tried to escalate and solve the issue. Ultimately, people are paying for these courses, and it's probably better to teach something rather than nothing.

They should not admit students who have little chance of success

There are several interrelated problems.