Originally Posted by tom tulpe

In the EU, they totally are allowed to do that, if it's a one-way rental out of the country, or if the road tax for the country has been paid. The latter strategy was used extensively by Buchbinder Autovermietung (where you were more or less guaranteed to get a car with Slovakian plates if you rented in Austria or Germany), the former is frequently the case at major airports. I had portuguese cars from MAD (one-way to LIS), German cars from LUX (one-way to FRA), and austrian cars from MUC (one-way to INN).