I don't see how the "Big Three" (GM, Ford, and Chrysler) have any say in what Toyota (or anyone else) sells in any particular market.
Yes, Toyota - like everyone else - sells their small trucks with diesels in almost every market worldwide. As Seef mentioned, in some they sell only
diesel (I recently noticed this is true for the Hi Lux in Germany). They certainly could sell diesel trucks here if they chose to do so.
The diesel engine currently used in the Ram 1500 (and Grand Cherokee) is made by VM Motori in Italy, the diesel in the Ram ProMaster
van is another (different) engine from the same source, the diesel in the upcoming Ford Transit
van is the same one used in the Transit van and Ranger pickup worldwide, and the coming (for 2016 model year) diesel for the GM Colorado/Canyon is the one from the corresponding worldwide version. Any manufacturer can take an appropriate version of their engines from other markets (presumably one meeting the most stringent Euro emissions standards), certify it for the U.S. (and hopefully Canada), and sell it on this side of the oceans.
I noticed that Nissan is making publicity noise about their Frontier Diesel Runner
pickup project with a small Cummins diesel engine... even though they have a different diesel in this truck in other countries, so they don't need the Cummins.
It's all about marketing and local economic and commercial issues. It will be interesting to see if, after all the hype, consumers actually buy these diesel vehicles as they become available. Most VW buyers don't choose the diesel in their cars, few Cherokee / Liberty / Grand Cherokee buyers have gone for the diesels that have been available in those vehicles, and I have heard that initial ProMaster sales are almost entirely of the gas engine.