It's really old "issue" which actually works as designed We probably used bad name for a component.. It's not a select actually. It's an input but with client side suggestions (as you see it allows to type anything even not from the list). So it supports only Value from selectItem and in general selectItem support was added only for simple declarative page definitions of suggestion strings.
In RF 4 we solved that by providing different component with the same direct editing capability - one pure select (allows choosing from the list only but allows direct editing for simpler filtering) and the second - autocomplete which unifies ajax suggestionBox and client comboBox from 3.3.3.
Thanks for your answer!