@Finn dAbuzz,
There are several issues here. There is the legal rights of players vs. their employers, and there perhaps are moral rights.
I think you are misunderstanding the power balance between players and owners. The owners are producing a product that depends on them attracting and keeping elite talent. They can't afford to piss off their employees, many of whom are not replaceable.
When I started working as a young software engineer, there were companies that insisted on a dress code (i.e. a tie). Now, I don't think such an employer exists.
This is because software elite software engineers have a great deal of leverage. If an employer demands their employees do something they deem unreasonable, that employer will fail, and the employees in jeans and t-shirts will barely notice.
The football players, like any elite set of employees, have a great deal of leverage in this case.