I'm thinking software. If I bought an application, in full, in a retail store. Then I go to install the product, and they tell me I need to accept their license. Can they do that? Even though I already paid for the product.
When you buy software, you have very specific rights given to you by law. You basically have the right to install the software on one computer, run it on that one computer, and make one backup. Each of these acts involves making a copy of the software, and you are allowed to make these copies (and no others) by law.
When you are shown a license, and are asked to either accept the license or return the software for a refund, you have three choices that are legal: 1. Accept the license, install the software, and do only things that the license allows (the license cannot take rights away that you have by law). 2. Don't accept the license and return the software for a refund. 3. Don't accept the license, and do only things that you are allowed to do by law. Which is never more rights than the license would give you.
Of course you might have bought the software under the wrong impression that you could install and run it on 100 computers in your company. You might then decide that the software is too expensive if you need to buy 100 copies for your purposes, and you have the right to return it for a refund.
I should repeat that accepting the license doesn't take any rights away that you had through the purchase. The purchase alone gives you very little rights, and the license may give you more rights.