What is the legal basis for fining players for out-of-game actions?

Question

How is it that sports leagues are able to fine players for things like expressing their displeasure with the league or its employees, or wearing the wrong clothes? There must be some sort of special employee-employer thing going on, because I can't imagine any employer I've ever had being legally able to fine me for legally protected speech. Is this something the unions agree to on collective bargaining? Is there standard boiler-plate language in every contract?

Update: I'm referring to the US-based leagues (MLB, NHL, NBA, NFL, MLS, WNBA) since they operate under US law. Leagues based in other countries may operate under different legal regimes and therefore may have different ways of doing things.

Answer 1

In the case of the NHL, this is definitely covered by the CBA, in particular section 18-A.2:

Whenever the Commissioner determines that a Player [...] has been or is guilty of conduct (whether during or outside the playing season) that is detrimental to or against the welfare of the League or the game of hockey, he may discipline such Player in any or all of the following respects:

[...]

(c) by imposing a fine on the Player not exceeding the maximum permissible fine under Section 18.7(b).

The MLB Basic Agreement contains similar language in Article XII - B:

Players may be disciplined for just cause for conduct that is materially detrimental or materially prejudicial to the best interests of Baseball

where "discipline" can include a fine.

For the NFL, "conduct detrimental to the Club" can result in

maximum fine of an amount equal to one week’s salary and/or suspension without pay for a period not to exceed four (4) weeks.

Source: Article 42 1(a)(xv) of the NFL CBA. Note that this is what may be imposed by a club; I don't believe there are any limits to the discipline that the league / the Commissioner can impose specified in the CBA.