I think the key word is "reasonable".
The IRS (which regulates tax-exempt status) allows a 501 (c)(3) nonprofit to pay reasonable salaries to officers, employees, or agents for services rendered to further the nonprofit corporation's tax-exempt purposes.
No hard and fast rules exist for compensation in a nonprofit, but the IRS can penalize both an organization and an individual for excessive pay. This expectation is embodied in the inurement clause governing nonprofit organizations.
https://www.thebalancesmb.com/can-nonprofits-pay-staff-2501893
And if a person who is qualified for a manager's position in the private sector could make, let's say, $900,000 a year with no problem, then it is safe to assume that a nonprofit can and should be able to pay a competitive amount for such a person.
A good example may be found in health care. There are non-profit hospitals and there are private enterprise ones. A good non-profit must pay as much for a director, heads of departments and other management positions as the for-profit ones or they will not be able to recruit the best people.
When the IRS says "reasonable" it obviously means "in line with industry or profession standards".
My mother spent about 30 years on the board of a large county public hospital system. They were given a stipend, or fee" which, if I recall how she explained it to me, was "at least 80% of the average comparable fee paid by other comparable health care organizations in the area for similar functions, obligations and responsibilities". They felt that community-minded members of the board might do the job for a bit less than private sector individuals, but they still used an index of that same private sector as the base criteria.
As a reference, during those years she got an annual stipend that would buy a moderately equipped Chevy or Toyota for work that involved 20 to 30 hours a week of document reading, conferences on the phone and physical meetings. And that was for a cash-strapped public hospital system in a county that was losing population and, except for one other hospital, hard-pressed to get qualified medical staff to go and live there.
So, getting back to radio, if we look at the public financials of EMF, their staff is rather significantly underpaid for what they do and for the size of their operation.
And the IRS sees each industry and judges accordingly. When is the last time they ruled against the salary costs of a non-profit? (Not a rhetorical question... I just can't find any example using plain old web search engines.