"Binding"... why not "Bound"?

[DarkmoonVael]

I'd like to chime in and say "Binding" has nothing to do with spiritbond. Bind and bond aren't the same word.
Bind Binding Bound. Bond Bonding Bonded.

To answer OP's question, I think it's intended to be "binding" like in the phrase "binding contract".

Items only become bound/binding to your character once the spiritbonding process has started. The words, as you say, may not mean the same thing. The process, however, is linked.

Binding, as in "binding contract", makes no sense what so ever sadly. You can not make a conscious agreement that can not be broken with an inanimate object, unless you were slightly barking mad yourself.