Sure, but it's Fillmore Graves, not the government who is enforcing this mandate. That was the crime their guillotine was built for.
First and foremost, they could be turned into zombies themselves and the purpose of this quarantine is to contain the zombie plague. Using US forces would increase the possibility of the zombie virus spreading. Secondly, thanks to zombism and their exclusive access to the Super Max serum, the Fillmore Graves mercenaries are faster, stronger, have better stamina and are all around more durable than a human soldier.
Is it though? They infected thousands of people against their will by purposefully contaminating a flu vaccine (for a deadly flu that they intentionally brought to Seattle!). If that's not bioterrorism, what is? I can hand wave the US government being willing to work with them for now until they come up with a better plan, but calling Fillmore Graves a terrorist force that has essentially taken control of Seattle is an entirely fair assessment.