The designs Amemiya did for the Black/RX episodes of Decade were original to that series; about the only cool thing Decade did, IMO, was get back original creature designers to do the designs for a new monster that went with the particular world of their show. (Like, Yutaka Izubuchi did the new Unknown in the Agito episodes.)
Amemiya's also done a couple of DVD package designs for Toei (like the Shin Kamen Rider cover art), so it's not like he has some feud with Toei -- I just don't think Toei's modern toku appeals to Amemiya's sensibilities.