surely it should be the commercial operators who fund the GA fitting of the Mode S trandsponders
Yes but that's not the way the world works
The entire aviation regulatory / administration apparatus around the world is geared for just one thing: commercial air traffic.
GA exists in the nooks and crannies, cracks and crevices that are left, which are for the most part safeguarded by ICAO. They are most definitely not safeguarded by national politicians.
In the few countries that have a significant amount of GA activity (the USA is big of course, and in Europe you have the UK, Germany, France) there is also support for GA through influential people doing private flying.
But that's about it. European GA could be abolished tomorrow and the Big Brother watching intellectual elite would never notice. Sad, isn't it?
Curiously, mandatory transponders have been an ICAO requirement for years. The UK CAA, despite being largely "owned" by the airlines, has protected GA from this by filing an exception and so have most other CAAs. For some reason, probably airline pressure, this is ending.