Ah, no, it looks like that was me skimming too quickly.

A closer reading indicates Bestushev conspired against Keith and caused him to lose his position of command, at which point he demanded his release from Russian service. He eventually got it, but then he had some friction with the authorities over the terms of his release: they wanted him to promise never to fight against Russia, whereas he was like, "Once I'm out of your service, I'm nothing more than a British subject, and I'll fight who I want." But since he didn't want to go to Siberia, he agreed to read the fine print. It said that if he did fight against Russia, he agreed to be judged by the Russian articles of war. He immediately agreed to sign that, since he was confident that there wasn't a single article saying a free Englishman couldn't fight who he wanted.

So while he did leave voluntarily, it was because he was pissed off at losing his command, and it wasn't on good terms. So much so that when he left, he took ship directly to England, since he was afraid of traveling by land and getting arrested.

Also, notice how he's a free Englishman, not a Scot. We've talked before in salon about the prejudice against Scots during this period, and when James is trying to quit Russian service, it's ~1747, i.e., just months after the defeat of the biggest (and final) Jacobite rebellion, the '45.