That "requirement" is meaningless.
Even if they never apply for permanent residency, it is impossible to prove that they did not intend to do so.
Also, on what basis would they apply for permanent residency? If they were eligible because of a family member, why wouldn't they have applied before?
They may have. They could have filed years before and have been waiting.
It takes many years.
The U.S. Department of State only distributes 226,000 per year for special visa (student would fall under this) and 140,000 a year for employment visas.
That is for the entire United States.
Plus there is a limit on each country.
If they do not apply - they cannot get a job or a drivers license so it would not make any sense for them not to.
They would just be wasting their money and their time going to college.