USE=static-libs is for controlling the *build* of static archives, not the install alone. Either remove the broken USE flag, or make it optional at compile time too.
Fixed in 2.6.2* ebuilds :) (2.6.1 does not have that flag)
This still occurs for tinyxml-2.6.2-r2 No logic to install .a: src_install() { dolib.so *$(get_libname)* insinto /usr/include doins *.h dodoc {changes,readme}.txt use doc && dohtml -r docs/* }