If you sell me something, you cannot tell me that I can't modify it for my own use, for example. You as the seller can't tell me that I'm not allowed to resell my textbooks, or to modify the frames on my eyeglasses, or to hire my friend to fix my vacuum cleaner.
Proprietary software licenses do do that - the main difference is that is a non-rivalrous good; I can redistribute software without losing access to it myself. Because of that, people think it's somehow 'wrong' to say that I should be allowed to purchase a program and then resell it to another person, but if you look at it the other way, that's no more 'wrong' than purchasing a physical good and then reselling it.
In the South, it used to be common to sell property on the condition that the purchaser never sell it to a black person (I think they still used the term Negro or 'colored' then). If I remember correctly, that was first deemed illegal in certain parts not because it's horribly racist, but because it violates the principles of first sale: you've sold me something, and now you can't tell me what I'm not allowed to do with it.
