There is actually a stretch of US 70 in New Mexico that is available without purchasing New Mexico. It was added with the update that added Clifton, Arizona as a proper city, although US 70 wasn't included with Arizona until partway through the beta.
I'm... not entirely sure why, though. They could have just continued US 191 down to Interstate 10, staying in Arizona while doing so. And if they wanted to make New Mexico more accessible, they should've added the western stretch of US 70 (between Globe and Duncan), which they never did. I'd like to hope we get this stretch before Texas, but I'm not really sure at this point... on one hand, Clifton was added two and a half years ago, but on the other we did get the "hole" in the middle of US 191 filled pretty recently.