The police certainly have the right to choose not to escort Dolphins players until they stand for the anthem. In most areas, this service is an off-duty service, much like a moon-lighting job in other professions. It is not part of their civic duty. Just like I can choose not to go to a game if I don't agree with team members' choices to kneel during the anthem, an officer can choose not to escort said team member. Every action has a consequence and we must remember that just like the officers can not, and will not, force players to stand for the anthem, those players then cannot force officers to escort them if they wish not to.
Yes, I believe that the police could ask people what and what not to do. They are tasked to protect the public, especially at a certain event. This decision is made regarding to everyone's safety at the stadium: the deputies, the players, the audience. You never know but a terrorist can be disguised anywhere waiting for a time to strike. Precaution is paramount.
The police swore to do a job, part of which is to protect famous people who might get mobbed and hurt when they are out and about. The police don't get to decide that someone exercising their right to peaceful protest then negates the responsibilities they swore to uphold when they took the oath.
No, I do not believe police have the right to chose who they want to protect. The police do not get to chose who they serve just because of a difference in beliefs or opinion. I believe the police have a duty to serve any citizen who makes a legitimate request to do so.