Is it common for ppl to confuse with "s" and "z"?

No, no native really gets confused. "Z" isn't used in a lot of our words, so "S" is something we are used to using... though, like someone else said, the "S" often makes the "Z" sound.

Sometimes in American English vs. British English, one is used in place of another, like "organization" (US) and "organisation" (GB).

I think some words in our language, however written with "s" are meant to have a "z" sound. Take a look "these", "is", "those". I think it's supposed to be sounded that way... iS instead of IZ or muSic instead of muZic doesn't sound natural to my ears...

Regarding other words (not all), I'd think it the same... You know that English language, you basically have to memorize everything. pronunciation, spelling. > _ < major pain in the **** even for us natives...

But to answer your question, I'd have to say... no, not exactly. :3 (as in some people might, but it's not common.)

Its more of a slang thing, but yah z gets thrown into alot of our stuff just to be there;) lol