People watch football to escape from politics. Viewers want to watch a game, not a protest.
Politics are inherently imbued in every aspect of life. What business practices your boss decides to invoke; what bathrooms you are allowed to use in public; every aspect of your life is governed by one law or another. To live is to participate in the political process. When you watch the football game, you support the NFL, a powerful organization with its own political impact. Most actions you take for granted have a political consequence. Hence NFL players' kneeling should be allowed.
The simple fact is that people are opposed to the manner of the protest because it is difficult to ignore. Arguments against it are examples of motivated reasoning: people look for reasons to avoid confronting something they don't want to confront.
The NFL made a specific choice after 9/11 to embrace overt patriotism. Once the emotional connection with many fans was made, they made a decision to keep using patriotism to advance their corporate interests. In doing so, they forced players into a symbolic action that many are opposed to. The NFL started this for corporate/marketing reasons. Players should not be forced to support actions that are against their principals for corporate profits.