Well, it's the difference between getting paid (salary, fixed, no logical incentive to grow/improve) versus getting paid (percentages of benefit, the more benefit you create the more you make, growing non-linearly with your actual output).
(postlude)
It occurred to me this morning that even in theory, market economies don't pay to the seller a percentage of the buyer's benefit. They never do: it's not how they work. The full area under the demand curve, above the line indicating price, accrues to the buyer. Buyers that get a very great benefit (tangible or intangible) out of buying a pencil, a gallon of gas, or a computer generally pay the same price as those for whom the benefit is very slim; and there is no limit to that benefit.
This is mostly beside your point, I know. I just thought it was a nice idea, worth sharing.
But, if a project is clearly the work of, say, 2-5 individual developers, it's easier to see how those individuals are creating much more value than anybody is possibly compensating them for.
I think this is true of anything worth doing.
I like your characterization of the mongodb model and the RedHat model. (I don't see anything wrong with RedHat's model, by the way. More power to them.)
But your complaint would have made a lot more sense fifteen years ago. Now there are a lot of other models. There are even other models that work really nicely!
There are consultancies, like Igalia. Some companies hire open source developers because they use the project and want to make sure active development continues (antirez's day job might be an example of this; at a guess that's why Google once hired GvR, the creator of Python). Some use OSS projects as a recruiting tool; smart ones use it as a signal of quality in hiring. If you selectively hire engineers with OSS on their resumes, you are unwittingly paying returns to OSS development.
Companies can also use OSS to reduce the cost of external efforts that are strategically important to them, and they can win a developer mindshare benefit from doing so. This is why Microsoft open-sourced the .NET platform (under a good license!).
I'm no VC but the bazaar seems healthier financially than it's ever been.
The strange miracle of open source software is that when companies use something like Redis to make a lot of money, they end up investing in it, they contribute work to the project, and the community does gain from their contributions. In other words, the whole point of OSS is that exactly the thing you say never happens, happens all the time.
Most open source development is commercial.
To take an example from my day job: Bloomberg (the financial media giant) sponsored Andy Wingo (of Igalia, that guy is a genius) to contribute support for ES6 generators to both Mozilla's JS engine and Chrome's. An example of his amazing work:
Companies use all things to get richer. That's the operating principle of companies. The competing model to open source is commercial software, in which the company owning the software has a monopoly over it and uses it to get richer... and the companies licensing it still use it to get richer.
In corporate welfare, the money given to companies is taken from someone else's pocket. Open source software is cheap because it eliminates the artificial costs of copyright monopolies.
(postlude)
It occurred to me this morning that even in theory, market economies don't pay to the seller a percentage of the buyer's benefit. They never do: it's not how they work. The full area under the demand curve, above the line indicating price, accrues to the buyer. Buyers that get a very great benefit (tangible or intangible) out of buying a pencil, a gallon of gas, or a computer generally pay the same price as those for whom the benefit is very slim; and there is no limit to that benefit.
This is mostly beside your point, I know. I just thought it was a nice idea, worth sharing.