>>I agree with that, we also used to be very careful with branding like 15 years ago, thinking that all kinds of people are trying to steal your sourcecode, but we have learned that if anybody would be able to understand and use our source code, we probably would make him a job offer instead of suing him.
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>>Software can be different than a book in such a way that it is used by businesses to run their production and/or administration. In some cases the software runs the core business and has a great part in maintaining or increasing the revenue of that company. So unlike a book or a simple guideline, without the software the company would not be able to make that money, so the producer of that software wants to have some revenue of that as well. A telecommunication company for instance runs their billing system on a software package. They also purchase and sell cell phones. The cell phone provider continues to make profit by selling their phones to the company, but the software provider has no material "thing" to send to the telecommunication company, so they charge based on transaction value. The larger the telecommunication company is, the more they need to pay for using the software. The software company needs this revenue to invest in updates and future development to keep their software on the latest standard, so it's a win/win situation, because in a few years the telecommunication companies can rely on more modern updated programs which have been developed.
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>>On the other hand, many foundations that do not make profit often receive free or very discounted software packages.
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>But that is a different argument, if the software company wants a stream of revenue for something as you described, like SAAS or similar I guess, is completely different discussion, in that case my example would have been that if you buy a book and you are able to make money with what you learn from the book (isn't it always the case? but I digress...), can the author take a portion of my earnings? I think that everyone here would say no, but coming to software we seem to accept that we, as writers of some software are entitled to the buyer's revenue? To me is terrible, at least the way I read your post.

Parties to a contract can agree to almost any terms e.g. "percentage rent" in retail.