While there are forms of Iaido/Batto Ho which indeed start from suwariwaza there are also forms which start from standing eg Munen Shinden Ryu. cheers, Joe.P
Ps Mike Flynn 6th Dan Shihan has produced an excellent set of dvds on Iaido. Anyone doing this discipline will be pleased by the content and standard of the waza being shown therein.
The way things were explained to me was that seiza was not a practical position for iaido ie. not one that would have been seen in everyday samurai life. But we practice it in the initial forms because it builds leg strength and form needed for the more practical tate-hiza (half seiza half cross-legged) which was the way you would have sat in armor (but difficult to start with), then move to standing forms.