1. Its referred to as the "piece concept" rather than the "weight concept". The piece concept generally applies to all int'l flights to/fr North America, as well as most transatlantic flights from Europe to the Americas (although BA just chopped this to South American destinations).

2) Its a cultural and legacy thing. Most Euro carriers were very small and very restricted/regimented, government owned. They saw this as a revenue source. Most Europeans until the very reent past did not fly scheduled airlines regularly except on business, for vacation they flew charters or drove.

3) On most carriers it is actually now 23kg, and even the strictest ones generally say nothing up to 30kg or so. Also, most top tier programmes allow an extra piece or 10-20 kg's.

4) Generally if you have an itinerary that encompasses both concepts, the piece concept wins out.