It’s not just RW, but anyone that contracts with T-Mobile. The roaming relationship is between T-Mobile and its partners. Because roaming is so expensive the carriers tend to do everything possible to minimize roaming.
A bit of a tangent, but related.
Over 5 years ago there were two carriers that drastically lagged behind the big 2 (AT&T / Verizon). These carriers were Sprint and T-Mobile. These two carriers took drastically different strategies in order to try and compete from a coverage perspective.
Sprint spent lots and lots of money allowing its customers to roam on Verizon. They got nothing for this money other than what they perceived as more customers and customers that stayed because they could offer coverage in areas where they never did before. Unfortunately, the amount they were paying to Verizon was so massive that it often replaced investment in their own network over the years.
T-Mobile decided that giving tons of money to its competitor wasn’t good for its long-term health. They knew that this might keep them in the 4th position by customer count for a while, but that investing this money in their network, in buying more spectrum at auction, and in marketing and customer acquisition would help them grow their customer base, loyalty and satisfaction in the long-term.
Fast forward to today. Sprint still relies on vast areas of roaming to provide its coverage. Because of the lack of investment in their network they’re routinely rated last of the major networks in coverage, speeds, customer satisfaction and just about anything.
T-Mobile on the other hand is a not so distant 3rd in the carrier race (customer count). Their native coverage has improved drastically, they are routinely rated number 1 or 2 in speed tests, have been growing faster than all the other carriers for many quarters in a row and now own vast quantities of low frequency spectrum which positions them very well for the coming 5G network build-out.
TL;DR: T-Mobile has a lot less roaming coverage than Sprint because they’re spending the money trying to build out their own coverage rather than paying AT&T inflated roaming rates that prevent them from investing in their own infrastructure.