There seems to be a perception in some that in cases where s and z are both officially valid (most of the time), that z is a vulgar Americanism (nasty pointy letter with a harsh sound, both of which stand out compared to the more refined smooth shape & sound of our lovely s!), when, as you point out, z is historically more correct in “proper” English. I don't think it is that they see s as english-english as such, but that they see z as american-english so go with s as an act of identity preservation.
I prefer -ise over -ize, I'm not sure why, maybe it fits better with how I pronounce/emphasise things when speaking. Though it isn't the case that either is more right in modern English: while -ize predates -ise, for about half a millennium it has been the case that both are valid so it is just down to preference.
Which ever you pick, just be consistent. There are some cases where one is considered valid and which one varies by variant of English, such as analyse/analyze, which is probably part of where the s-is-English comes from – applying a specific exception widely as if it were a rule.
There are a few things that many are convinced of about English which turn out to be quite false: the singular “they” being another one that has caused much discussion in recent years, or the not-splitting-infinitives thing which some are still determined to boldly hold on to.