Did you look for images? They torture the bulls by attaching burning items to their horns. Running away is the least significant part. It's a very cruel activity that end up with a lot of dead bulls.
Apart from the animal abuse issue (which is also what I care most about), it's also a cultural issue. If Catalans consider bull running to be Catalan heritage, but not bull fighting, they might allow the former and ban the latter. But Spain considers both Spanish cultural heritage, so wants both to be allowed anywhere in Spain.
BTW, I also hate bull fighting.