SOAP APIs tend to use POST even for readonly methods, and tend to use aggressively validated XML that's much harder to write by hand than typical "REST" JSON.
I'm sure why using POST for readonly methods is a problem in the browser. It potentially messes with caching, but it should still function correctly. (Granted, this is probably a poor design design decision, but has nothing to do with correct functionality.)
As far as constructing the XML, you should be able to use something like xmlbuilderjs[0]. That said, I completely agree that dealing with REST APIs in the browser is far more practical.
