Have you actually done that?
There's nothing particularly trivial about it. Even if software calculates the number this means that each small business will have to remit payments at least quarterly to 50 different authorities. That's a major pain in the ass. Even if it takes less than an hour per state, that's someone's full time job for a month of the year.
The consider that some states tax shipping, most don't, and I believe some states even tax free shipping at the actual value. NY doesn't tax clothing under $100. Georgia doesn't tax energy efficient products between Oct 5th and 7th. All kinds of states have exemptions for school supplies, but I'd bet they don't consider the same set of items as "school supplies".
Plus if this goes ahead, then county sales tax will surely be fast on its heels. That get's into extra special levels of stupidity - in the town I used to live in, you only had to pay for the transit district if the land your house is on was annexed by the city after 1992. Even ordering stuff from the national retailers online, most of them just gave up and asked me which tax rate applied to me.
Don't get me wrong, i have no issue with sales tax as such, but it needs to be simple or it'll really hurt small online retailers (I suspect it'll actually be a win for Amazon). I'd rather see something like a flat 5% or 8% that your remit to the federal government and they do the work of dividing it up.