I don't think (1) really applies. It sounds like the vaccine is very cheap ($20) and you simply don't take the boosters if you're not returning. At that price, the significant side-effects of most antimalarials are more important considerations.
I think you make a valid point, although I was able to acquire a course of Chloroquine to cover me for a couple of months at one week's notice, for <£10, whereas the vaccine would need two months for the three initial doses. So maybe it's a practicality argument that I'm making, rather than a financial one.
