Why are some non-thesis MS or MA programs just called M?
In general the formal name of a degree is a very bad guide to the contents of the degree; the name of the degree is heavily influenced by local history and traditions (some extreme examples: a Master of Arts degree from Oxford is automatically awarded 21 terms after matriculation to anyone with an Oxford BA; the degree that qualifies one to practice medicine in the USA is a Doctor of Medicine, while in the UK it's typically a combined Bachelor of Medicine and Bachelor of Surgery; a Bachelor of Philosophy degree at Oxford is a two-year philosophy degree aimed at those with a first degree).

In the UK there are increasingly degrees called 'integrated masters' (or sometimes 'undergraduate masters') whose names fit the pattern you mention. These are generally four-year degrees, generally in fields like mathematics, science and engineering, which one enters without a previous degree and which aim to leave students in a position to embark on doctoral study. See this example from Manchester Metropolitan University. Individual degrees may or may not include a mandatory or optional thesis requirement.