I learned to do it because old-school unix vi would identify your sentences better for sentence-based moves if you had 2 spaces after the period. I'm actually trying to unlearn it now because you don't need it for any kind of vim/neovim and I read on some ultrapedantic typesetting website that it's wrong for some reason I don't quite remember now.
In any case it's definitely not specifically American (neither am I) and all the other "forensics" that people are trying based on vocab etc are somewhat of a stretch also. eg non-Americans use "gotten" non British people use "British" English (eg people from Ireland or from former British colonies for the most part) non-Americans use "ize" sometimes for spellings (I could never be bothered to learn the few exceptions needed to spell "ize/ise" words correctly in the British style and worked enough for American companies who wanted US spelling as a house style for my personal spelling to be even remotely consistent and certainly not indicative of where I am from.
As a British person, despite having been taught that the '-ise' suffix is 'proper' English, I have made some effort to unlearn this habit, as it was never really based on any etymological roots anyway. Here's what Wiktionary has to say about it[1]:
Many English verbs end in the suffix /aɪz/. Historically, this has been spelled -ize on words originating from Greek (for example baptize, Hellenize), while -ise has been used, especially in -vise, -tise, -cise and -prise, on words that came from French or Latin roots (for example surprise, supervise). In the 19th century, it became common in the United Kingdom (due to French influence)... to use -ise also on words that had historically been spelled -ize (hence baptise, Hellenise). However, the... Oxford English Dictionary continue to use the spelling -ize on Greek words, and -ize has always been the spelling used in the United States and Canada on such words.
The whole debate becomes rather moot when it is considered that Ancient Greek didn't use the Latin script, so both '-ize' or '-ise' would have looked distinctly foreign to a Greek author two thousand years ago. Horace so succinctly noted that "Captive Greece took captive her savage conqueror and brought civilisation to barbarous Latium", but he might have been a little less glowing if the orthography of Greek loanwords was as heated a debate in Rome as it is in contemporary Britain!
