You thought right. Well, kinda.
Conditions dictate maximum speeds, in the judgement of the citing officer, not by the offending driver.
And there are speed limits after dark.
So, although it sounds like there aren't speed limits in Montana, there are. You just don't know how fast someone else thinks is too fast. And, it's best to keep it under 70 mph in the last week of the month.
Those non-existent quotas for traffic tickets, y'know.
OK, thanks.