There is a quid pro quo in open source. It's not always in the contract you click agreement on. But even in the loosest BSD-style contract, it's implied. You stand on a lot of shoulders, and your additions must become part of the structure.
The principle derives from many sources. It's at the heart of the scientific method, the idea that we stand on the shoulders of giants and what we do will become part of the civilization our children and grandchildren live in.
Perhaps the problem above is with the word our. It can mean, only our direct progeny. Or it can mean, all descendants, all those who will come after us.
In a corporate sense we usually think of that more restrictive meaning of our, especially when we're dealing with what's subject to copyright or patent, so-called intellectual property. And it is this meaning of our which open source most directly attacks, either explicitly (as Richard Stallman sometimes does it) or implicitly (as Sun has learned to do it).
When you look at criticisms of both Stallman and Sun, you usually find cynicism as to motive. Stallman is caruicatured as a latter day hippie. Sun is seen as having something up its sleeve, as hiding something in the fine print.
In the proprietary world, of course, you don't have to think about such things. Pay your money, click the EULA, and no one is asking you to love Microsoft.
I guess, as Bill Clinton might have said, it depends on what the meaning of our, are.