The rule at play here is:
A licensee may not, directly or indirectly, restrict access to any portion of an establishment wherein gaming is conducted, through the assessment or imposition of a fee, except upon receiving prior written administrative approval from the chairman consistent with policies of the commission. — Nevada Gaming Regulation 5.210
As far as I know, the only exception Gaming has ever given is to the Playboy casino, who indeed charges admission to patrons who do not meet any criteria for free entry. My source at the Hard Rock says they applied for an exception but were denied. They speculate the main reason was that Rehab charges different prices depending on the day of the week, among other factors. Evidently Gaming wanted to see a consistent pricing policy to get an exemption.