They're doing a huge amount of work. They're going to write portale code because they are good programmers but not going to to the porting---just like openssh. They write good solid portable code, other people port it, everyone wins.
Most peoples' definition of "portable code" is that it's, you know, portable. It runs on multiple platforms. Write once, run across all substantially-similar systems. For example: Unix utilities running on the POSIX platform are portable because the exact same unmodified source code can be compiled on any POSIX platform against the standard POSIX system headers and linked with the standard libraries and run. Much portable code also has OS specific performance enhancements: it may take advantage of an OS facility that is non-portable if available. Non-portable code is written in such a way that it must be modified to compile on other operating systems using standard, portable interfaces--a non-portable OS facility is used in all cases, and if not available then you cannot compile the code.
Your fallacy: Equivocation, the informal logical fallacy of calling two different things by the same name. In this case, "portability" (the ability to simply carry one thing from one place to another--in programming, the ability to compile unmodified code on various platforms which supply a standardized API) and "porting" (the act of making a thing portable--in programming, the act of rewriting non-portable software to be more portable by making it compile on additional platforms).
It's funny hoy you cite "eonomics" as your argument for why people should give you free stuff.
Yes. It's called wealth production. You see, if you use 1 unit of labor and produce 1 unit of output, you have created 0 wealth. If you use 2 units of labor and produce 1 unit of output, you destroy 1 unit of wealth. If you use 1 unit of labor and produce 2 units of output, you create wealth.
As I've explained, it takes some units of labor (effort, work) to fork a code base, greatly improve it in a way which makes it non-portable to the platforms the original code base was portable to, and then apply additional labor to modify the result to again make it portable to the same original target platforms. It takes some fewer units of labor to simply retain portability as you make the improvements. The end result of both of these strategies is the same; however, the second strategy requires fewer units of labor input--it destroys less wealth in the process of creating the same wealth output, thus it is economically more efficient.
Think about if you paid $10,000 for a car, then paid $1000 for new tires and $3000 to add a V6 engine to replace the I4. Now consider if instead you paid $12,000 for the higher model which comes with the upgraded tires and the V6 engine. In both cases you get the same car; however, in one case you get it for $14,000 and in the other you get it for $12,000. In the first case, additional labor is used to install, ship, and then remove the original equipment, which is then replaced with new equipment which must be installed and shipped. The first install-ship-remove cycle (and any re-shipping to get those parts to another place where they are useful) is avoided by doing it right the first time, which is where the $2000 savings in this example comes from (we assume in this model that the automaker uses a static margin model, where everything is produced and then has a certain marginal profit slapped onto it).
Why would you waste effort making additional work?