As you have noticed, whatever you type into Keywords tab, it is treated as "single" keyword. If user enters "single" keyword, which contain comma(s), then this may (and usually does) cause troubles when trying to change/remove that particular keyword.
I have hesitated a long time before implementing "keywording", and finally decided for "basic" approach -which should (at least) allow some keyword editing. Right now, I don't think to make improvements in this regard, as this would not only exceed GUI's main purpose, but would also require a lot of work (to do it "right").
My personal opinion (from visiting various forums)... Many are not aware of meaning of term "keyword" and it's usage.. Basically, Keyword entry should contain single "keyword" -not "keystatement". And then, Adobe implemented "hierarchical" keywording... which is, IMO, not only useless, but also makes maintaining (reorganizing, transferring, etc.) keywords later, much more difficult.