That is pretty much the definition of freedom of speech universally applied. You are free to say what you want and your employer is free to say things like "you're fired."
Freedom of speech != speech without consequence.
Freedom of speech != speech without consequence.
By that argument the Weiße Rose in ww2 Germany had free speech but they still got beheaded for it.
the UNHC defination is "Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers."
Note the "without interference"!
Governments throwing you in jail, intimidating you, or out right killing you all disrupt your right to free speech.
An employer exercising his free speech by firing you in no way prohibits your free speech. You can still tell everyone you want that you should form a union. No one is interfering to keep you from doing so.
Please tell me you don't actually equate those two scenarios.
consequences != interferences
Another example is the Chinese allowed protestors against the Olympics in special zones - they of course arrested them after - that is not free speech.
Possibly you ought to update the constitution in line with the Universal Declaration Of Human Rights like most of the first world countrys have.
Most people don't know that in some jobs it takes half a year or an entire year of salary to train someone new to their new job until they start being productive to the company.
It is also something employers do not love to do, firing people is hard emotionally. Most of them are not sociopaths.
How do I know? I had been and I am "employer" myself.