In regards to realism: There is no Federal limit to how fast trucks can go in the US. However, a lot of companies choose to add speed limiting devices (governors) to trucks for fuel economy. The governors are typically set to around 65 MPH.
In regards to gameplay: Setting a global speed limiter in cities doesn't make much sense in ATS. ETS2 Europe #2 it makes sense, but not in ATS. There just isn't enough traffic or issues with people causing issues for this to be a concern. If it ever does become an issue, then it should be considered, but until then I feel the limits should be removed completely. The roads in America tend to be far less curvy and the trucks themselves are less prone to tipping over than in Europe.