Foster's assumption, somewhat reasonably, is that the bathroom is the easiest part of a house to clean up. Tile can just be mopped, or bleached, and you don't have to worry about things seeping too deeply into or under the carpet.

He's already dripping blood into the carpet, of course, but it's not like that was pristine to begin with.

He considers his handfuls of restraints, decides to carry them with him just in case--and follows her obediently to his own white-tiled bathroom.