Definitely unethical and illegal for a child/student to claim his own dependency, get EIP/RRC, etc., IF HE CAN BE CLAIMED BY ANOTHER. As you point out, a high earning child could be providing more than 50% of his own support, so it is possible, ethical, and legal IF YOU KNOW HE CANNOT BE CLAIMED BY ANOTHER. Have parents and child fill out a support worksheet with you, and keep it in your files to document the outcome.
I have one coed angry with me, because "all her friends are doing it." But her parents are happy to keep the education and other credits, and they DO qualify to claim their daughter.
Don't forget that the student can get the AOC WITHOUT CLAIMING HIS OWN DEPENDENCY if his parents do not claim him, even when qualified to claim him. He won't get the refundable part of the AOC, though. I have high-income parents who can't claim education benefits but have a college kid with a tax liability who can use the nonrefundable AOC. That is perfectly ethical and legal.
We don't write the laws, but we do have to work within them.