You raise an interesting points with regards differences in europe. I suppose there is rightly a much greater common denominator between class a (and tmas) and "international" commercial airports throughout the world enabling much the same standards of air law to be applied. Differences are far more common at small non commercial airports, and ifr traffic very rarely has to worry about airspace and other local perculiarities. For that reason demonstrating familiarity is perhaps even more important for pilots of ga vfr traffic than cat pilots.
Never the less easa ga pilots are free to roam although as you rightly say the organisation of airspace and local procedures can be quite different throughout europe. In fact of couse this was true before easa. From europe's point of view like it or not i guess it would be impossible to impose the need to pass an air law exam for each member state and i also guess easa would say thats why they intend even greater harmonisation.
On the other hand i equally guess australia would say the way we do business is sufficiently different that we are going to impose an air law requirement on foreign pilots.
Whether they would be justified is more difficult. We all know you can just go fly in america under a 61.75 but you do need to pass a flight test and you will (or should) be closely questioned on many of the essentials of air law. There is undoubtedly plenty to catch many the private pilot out in terms of the differences between the way they and we do things. On the other hand we give faa pilots (at least at the moment) complete freedom without having to demonstrate any knowledge what so ever of our air law. You could argue we are more generous. Is there any evidence a pilot is better off for passing a local air law exam? I have no idea. I guess it forces everyone to at least review local procedures sufficiently to pass whereas without while the vast majority will have the common sense to do so its surprising how stupid a small minority can be.