if I grow bannanas and sell them for $10 dollars I might sell some. If you, (copying my business model, my product and service) grow bananas and sell them for $8 - assuming all other things to be equal - you sell more bananas. Should this kind of copying be outlawed? Of course not.
If you don't understand this, you must BE bananas
The fact that some publishers PROVIDE copyable material explicitly for that purpose surely only supports the argument that being copyable is a feature. In fact, every feature added to a book costs extra. Maybe the people just want simple books with less features for cheaper than it costs to get them copied. If you only wanted to sell more books, this would not be a problem.
Perhaps we to easily forget that the value of a given product is only worth the amount of labour I want to save. I only buy a text book so I don't have to write it myself. Sometimes, it is much better to write it myself. People get stuff copied because its not worth them buying the original. They are going to read it once, draw all over it, etc.
Value is only the labour the customer thinks is worth exchanging their labour for. Otherwise, they just grow their own. Even if I carry a given box of bananas five times around the world, although in my view those bananas represent a hell of a lot of work, it means nothing to anyone else.
& lets face it. Textbooks are usually just recycling the same old grammar patterns, etc. It's not like novel, say, which one can enjoy again and again.