“Employers have wide discretion when it comes to limiting the political expression of employees in the workplace. The First Amendment generally applies only to government censorship of speech. As such, the Constitution allows private companies to regulate speech, even to bar political discussion entirely...
...In general, regardless of a particular state’s rules regarding political expression in the workplace, an employer can discipline or discharge an employee for legitimate, business-related reasons, even if the conduct involves political expression at work. For example, if an employee’s political expression interferes with his/her work, disrupts his/her co-workers, or infringes upon a business objective, the employer can take action consistent with its written policies and practices.”
In this case, the "employer" is the San Francisco 49ers football team. They can do whatever they want with him. What do you expect them to do -- bench him? They've already done that. He's a fifth-rate quarterback who lost his starting job and never should have been signed to his stupid contract in the first place.
What else can they do -- cut him? They're likely to do that anyway, but they have to pay him for this season regardless of what they do.
Ignoring him is the best course of action, because anything else they do would make a martyr out of him. He probably has an IQ of about 85, and it really shows. Anyone who shows up for an interview in a Fidel Castro T-shirt and denounces "oppression" in the United States is dumber than a sack of hammers.