Interesting, when does A:R state that?
Maybe by 2210 (the approximate date of AVP 2010) the USCM was no longer a unit of Goverment but had become a private entity. And perhaps in 2349, when megacorporations such as Weyland-Yutani were outlawed altogether, around the same time the USCM were forced to only have defence contracts.
Just for reference the majority of Alien Resurrection takes place in 2381.
On another note Alien: Invasion marks the event of W-Y buying out the Colonial Marines as happening at some point in 2600 AD. The company had become vast and powerful enough to become a major politcal influence rivaling official government.