I rarely ever agree with Rush Limbaugh, I think he is a overly pretentious clown and a hypocrite. However, he is correct in this particular case.
The NFL is a business, the team owners are the business owners, and the players are employees. There are laws that govern workplace behavior, for example: an employer may not discriminate against employees on the basis of race, gender, religion, etc: an employer must provide reasonably safe working conditions by observing certain safety rules; and employers must pay employees a certain minimum wage. The government is justified in creating these sorts of regulations, in order to ensure employees are not taken advantage of by unscrupulous or unethical employers.
President Trump made the statement that NFL players that kneel during the National Anthem should be fired by their team owners. It is an extremely dangerous precedent that Trump is setting by publicly involving himself with the personnel decisions of the NFL owners. The "government", including the president, Congress, or any other public official, has no business telling businesses how to manage or discipline their employees; beyond the established general regulations. This may seem like nothing more than Trump expressing his opinion on the matter; but as the President, Trump is not like other citizens. His words can have real consequences. As a matter of fact, most presidential speeches are carefully screened so a president does not cause an inadvertent public panic or some other sort of calamity. What did Trump accomplish? His words caused the NFL teams to circle their wagons and present a united front against him. The NFL is a wealthy organization, with billions of dollars available to wage a public relations war. What could have happened if the target of Trump's ire had been a small business that lacked the financial muscle to stand up against public outrage and the constant media bombardment?
Was Trump justified in attacking the NFL? Perhaps if he was a private citizen, but not as the president. The NFL players have Freedom of Speech, and they can protest, kneel, sit, or lie down as it pleases them. However, there is a time and place for everything, and the football field is not the right place for players to be protesting. The public is expressing it's outrage against the NFL by not watching the games on television or attending the games in person. Ultimately, the NFL is most concerned with the opinion of the public. If the public gets outraged enough the NFL could face a real problem.