I’m opening a bookshop that sells physical books. I have a machine that scans physical books and converts them into digital books. If a customer buys a physical book but wants to get a digital version for free, he can give me his physical book, which I store in my shop, and I can give him the digital version, but if the customer wants to get the physical version of his book, I have to take back his digital version. The only transaction the customer has to make is to buy the physical book, whereas nowadays, physical books are sold separately from their digital versions. Does my bookshop comply with the law?
You can’t own two copies of the book at the same time: that’s the whole point of the concept. Yes, you can make copies of the physical book, but even today that’s not very important - the book industry is doing well. What’s more, you can imagine a bookshop that offers a two-way street: you like having an e-book but then you find it good enough to buy it in physical form. In that case, you add a few dollars more to get the physical version.
You can trivially copy and ebook though, so can’t prove deletion.