They were not these person's "clients", they were the government clients of the company employing him/her. I would have expected the US employees to still have the freedom to say "fuck you!" to their government, even though said government might be on the list of one employer's clients.
More generally, one shouldn't have to give up his/her personal rights to say "fuck you!" to the government once he/she enters an employment contract.
Just because you retain the right to say F U to your government doesn't mean you have the right to say F U to your employer's customers while acting in the context of such employment.
They absolutely do have that right. They even exercised that right. The government did not send police to their house to arrest them. There are many governments that would.
What they don't have is freedom from consequences. They don't have imnunity from being fired.
And perhaps that is a problem in a world where it costs an individual a lot if they are fired, and it (typically) costs a company much less. The result is a chilling effect on the right to criticize the government via corporate punishment, and just because it's technically legal or laundered through a corporation's actions doesn't mean it's a good effect to let continue.
More generally, one shouldn't have to give up his/her personal rights to say "fuck you!" to the government once he/she enters an employment contract.