Personally, while I would not penalize an employee for political protests on their own time, I would not permit any employee (if I owned a business) to engage in any political protest on company time, regardless of the manner or purpose. But I would make that determination clear to the employees in advance of attempting to mete out any punishment therefor.
I don't see it as right to fire people for protesting, unless there is a franchise/league decree in advance stating that on-field protests are considered insubordination and a violation of rules/policy. In this case, they're clearly not, so employment-related consequences would be unreasonable.