The nature of the law enforcement practices in the Imperium seem to be nebulous. The game itself posits and interesting juxtaposition of a strong central authority figure (i.e. the Imperium) that is both crushing in its' enforcement of laws, but also quite laisez-faire in its setting of said laws.
The quote from Dune of "the spice must flow" is apt here - except one should substitute 'trade' for 'spice'. The Imperium allows worlds a great deal of latitude, as well as individuals and corporations, to do all sorts of things against each other and 3rd parties - so long as trade and the economy are not affected. Which is an interesting dichotomy as certain activities will naturally affect economics at all kinds of levels. One cannot overthrow a planetary government, or even mining colonies or whatever, without causing short or long-term disruptions to output. I don't think it's very reasonable to assume that Party B overthrows (violently, as it's Traveller) on Day 1 and on Day 2 everything is back to normal.
So one would (or could/should) assume that these sorts of things MAY happen, they probably aren't all that common. SuSAG is probably not going to work with Interstellar Arms to wipe out an LSP manufacturing facility just to get a corner on the market. The RPG Shadowrun has a background where actively taking down opposing corps is the norm, however I've not really gotten that impression from Traveller over the decades. Sure, you may see a Hammer's Slammer scenario where oppressed colonists hire a merc company to take down their oppressors - but let's be honest here. Assuming that happens, and there is enough credits, the megacorp is gonna be back with it's own mercs to reassert control. And they'll have more money and incentive to put the colonists in their place lest word gets out and it causes a sub-sector or sector-wide 'Freedom' spring to occur.
Which I take to be that these sorts of things are not the norm. And that the Imperium isn't a repressive police state with a blind eye towards terrible governance so long as trade isn't affected. These things are all interelated because at the bottom of them all is the people of the worlds that make up the Imperium. Some sort of social stability has to be present in order for this to occur. As we have seen in history there are many paths to stability, and not every path is one that people would like to live under. At some point every authoritarian regime has fallen - either to outsiders or to internal conflict as repression tends to beget rampant corruption.
I think there is enough wiggle-room to make for an environment of adventuring without a need to create something that shouldn't probably exist for as long as the Imperium has.