I'd question the assumption that Europe is "being naturally conducive to one-way trips for business or pleasure." For most Europeans (who, let us face it constitute the overwhelming majority of car renters in Europe), it would not seem natural to hire a car in one country and drop off in another. Rental cars are rarely hired for long distance travel. The market just is not there. Do not forget that Europe tends to have a relatively dense train/public transport network and for the kind of distances Americans would typically rely on a rental car, Europeans are much more likely to rely on rail/public transport.