If you're referring to mercenaries for "morally flexible for money groups"-
The MCC officially has taken a hardline stance against corporations and corporate power. Realistically, corporations provide too much of the domestic product on Mars, pay off too many officials, and are simply too profitable to really act against. It would be look shooting one's self in the foot. Additionally, because the corporations essentially handle the civilian market by themselves, the government is left free to essentially invest everything into rearming and industrializing. As such, corporations act with impunity and with few regulations on Mars. Any official effort to curb their power is immediately met with bribes, lobbying, and even assassinations. Simply put, the corporations do their own thing and the MCC does it's own thing; the two hardly ever interact.
As for the UN, corporations have significantly less power, if at all, as the UN/UNCA has nationalized most businesses and industries. With heavy regulations and taxes, corporations are only able to eke out a modest profit from Earth. This isn't to say that they don't have any power; as they have numerous politicians under their payroll. However, with the UN investing more into watchdogs and surveillance, corruption is a risky business. The UN's currency, additionally, is more regulated and of much higher value than Martian scrip, making business on Earth additionally more complicated. Tl;dr corporations don't really like the UNCA or find it as profitable as the Frontier.
As for corporate and government relations with mercenaries. Governments find them unpleasant to deal with, but a useful tool nonetheless; as easily deployable forces that operate without a flag or any connection back. They're often hired to take care of bounties, or small-scale, but important missions that are considered either too dangerous or insignificant for foreign intelligence or militaries to get involved. Otherwise, they're viewed only slightly higher than pirates; the only difference being that they use their guns for money instead of loot. Corporations, while officially feeling the same way, make heavy use of mercenaries when the use of their own private security forces would be much too overt or potentially damaging to public relations. Corporations hire mercenaries all the time, especially to conduct business with illegal parties or outside the sphere of corporate influence, such as in the more remote colonies further out on the Rim. Of course, whenever a group of pirates go too far and make off with a shipment of merchandise, mercenaries are always willing to sling lead and get the goods back without asking questions; which makes them very appealing to most private entities.