it's only much later games that used region lockout. and a few PAL games that were written specifically for 50hz.
Most games however were formatted for NTSC, and when released in EU, we got them slightly squished, and running slightly slower. I have lots of genesis carts, and JPN MD carts which run just fine on my PAL megadrive with no modifications.
On the other hand if i do mod the MD, i get to play them in 60hz, at the speed they were originally intended, and full screen. I really should get around to doing it i guess.