Remember people, that just because a British subject renounced British citizenship to become an American, didn't mean the British accepted that.
That's true to this day. Canadians, for instance, who are viewed as citizens be the country they travel to can be imprisoned, for instance, and the Canadian government can do nothing about it, really.
My brother's in-laws never dared go back to Latvia until Gorbachev came to power in the USSR, because they would never have been allowed to leave again.
And, more recently, there's been issues with Canadian/Iranians not being allowed to leave Iran after visiting. (Or mothers with custody can't get their kids back when Dad takes them to Iran).
So, this is still accepted international practice.
Britain was ENTIRELY within her rights to press British sailors on US merchant ships.
The problem arose when you 1) had disputed citizenship, and 2) that the US encouraged fake documents.
Given both of those, it's no wonder that British captains had to 'use their judgement', and occasionally snapped up people who really, genuinely were US and purely US citizens. Whcih did happen, but was far, far less common than you'd ever believe from US history books, or newspapers at the time.