Well it´s because you can´t be allowed to build higher stage ports, which allow gunpowder ships to be recruited before gunpowder is invented, not sure were they restricted to either city or castle, ports i mean.
From other nations buildings you get bonuses and benefits from all there is, but you can´t recruit specialist troops from there, such as Teutonic Knights as any other faction than HRE, or Knights of Santiago as any other expects Spanish and Portugese. If it´s high enough stage chapter house, you WOULD get the +experience benefit to those knights, if you could recruit them ^^