But someone still has to define what it means to have "part of your business" in a state. You own fixed property that's located there? One of your employees works from home and lives there? One of your employees worked 2 hours while waiting at an airport in that state? Someone rents something from you and takes it into that state? Your cloud provider keeps a backup facility in that state?
It seems like the definition of what it means for a business to have nexus in a state (so, what business counts as "interstate") is a perfect thing for the feds to handle.
It's referred to as having 'minimum contact' with a state; it's not just for tax purposes, but would also matters for things like lawsuit jurisdiction and so forth. Here's a recent summary with good references that explains the basics: http://www.picpa.org/Content/41763.aspx
It seems like the definition of what it means for a business to have nexus in a state (so, what business counts as "interstate") is a perfect thing for the feds to handle.