Use tax is arguably unconstitutional due to the interstate commerce clause, and that is why states do not enforce it. They can wield the moral force of "this is the law" to those that don't know better and get them to put it on their tax returns, but they won't go after those who don't pay because they're afraid to lose. The states' end game has been a federal authorization for the states to collect sales tax because it would put them on much more solid legal ground.
You've clearly never been through a sales/use tax audit as a business.
They do not feel like it's unconstitutional, and are not afraid to enforce it. it's not a "moral force" - it actually is the law. You cannot get out of state entities to collect sales taxes for you, but if they can show that you have nexus in a state, they can make you collect them. If you buy things from anywhere, in or out of state, as a purchaser you must pay use tax on it. It's not a suggestion for the ignorant - it is the law that you report your untaxed purchases.
Technically you owe this on everything, even those things that have been taxed by other states - it's just that most states agree to reciprocity.
There is very little gray area here. As an individual they probably won't go after you, but businesses that are supposed to collect sales tax (including those without a physical presence) and pay use tax - they go after you like wolves.
A universal rule for everyone would be a dream for those that process sales taxes. It's ridiculous the amount of time that is spent figuring out which sales are taxable, what jurisdiction those are in (the state, county, city, LOST, misc taxes) and how to report and pay those.