Premise: Bucky, living in the Tower, made sure that part of the conditions for working with and for the Avengers meant he got to have his partner living there with him.
This was doable from a legal perspective as surprisingly enough there was very little mention of Brock Rumlow by name in the Hydra file dump. By the time Rumlow was fit to be charged and stand trial he ended up getting either a mistrial due to gross incompetence in bringing charges without substantial evidence that would hold in a court of law, or acquitted for the same reasons.
Brock Rumlow’s greatest asset was always his ability to bullshit. He was trained by the best, after all.
The fact that anyone that could bear witness against him was either dead or severely emotionally compromised more than helped his cause.
Anyway, Brock Rumlow is a “free man”, however free you can be with Avengers stalking him on the regular. (He has fun getting restraining orders against them even if they don’t last for long.) When Winter comes back in the picture they all get a little bit of what they want. Rumlow gets reunited with the only man that has ever been in his corner, Bucky gets back the only good thing he had in Hydra, and the Avengers get to keep Rumlow where they can keep an eye on him.