Because when you assign a visible colour to a particular invisible wavelength, you can choose whatever colour you like. If you went there and looked with the naked eye, you'd see just black.

i don't think there is a single instrument on Hubble which is confined to the human visible portion of the electromagnetic spectrum, or any subset thereof. and i'd be amazed if duplicating the color response curves of the human eye was a design consideration.

unlike your DSLR (unless you popped the IR filter off of it).