Why does the Discipline of the Bachelor Degree matter for most Non-US Architecture Master Programs but not for US Architecture Master Programs? After some research, I've noticed that in the US it's mostly possible to apply for M.Arch programs regardless of the Bachelor discipline while outside the US a Bachelor in Architecture is required. 

Why is that? 

The master in the US takes 3 years for non-B.Arch people while in US/Europe it takes 2 years for B.Arch people. 

Can one additional year make up for the 3 to 4 year Bachelor degree?

Or is this whole idea just a marketing-gag of the US universities?

(And if anyone has some experience: In general, how successful are non-B.Arch people in M.Arch programs?)
In the US, a bachelor's is almost always half or more liberal arts - that is, a bunch of different subjects outside the major. It is run in such a way that basically little more than literacy and slightly more than arithmetic are assumed to have been developed in previous education. 

For example, a student often has 4-5 classes each semester for 4 years. Of those, only 1-2 are usually in the major each semester, though some majors start with 0-1 classes in early semesters to "get the general education classes out of the way", then later semesters are more focused. This is one of the reasons double majors are not so rare here - if you can count some of the other majors requirements as "general education" (you often can), and take electives in the second major, you can end up managing to do both majors without taking much longer to earn the degree.

At the master's level, the extent to which a closely related bachelor is preferred varies, but is rarely required regardless of subject - only at very picky departments is it a big deal. 

Is 1 year of focused master's education a replacement for having a same-subject master's degree? It isn't a perfect replacement, but mostly yes. It will tend to be more demanding and hard on the student as they have to play catch-up, so this is not as attainable for all students - but it is well within the realm of human possibility, especially when motivated by the fact that all the coursework is much more likely to be strictly necessary to support future courses. There is a lot of optional "not that hard to figure out on your own if you actually cared and had a use for it" stuff in higher education, after all.

I personally know some CS and art/design people who've done MS in architecture in the US and they seemed to do fine. I'm sure it is harder the less applicable your previous education is, but it is not at all an alien thing here to switch subjects like this.