Why did they allow different brooms to be used in Quidditch? In Harry Potter and the Sorcerer's Stone, it seems like the Nimbus 2000 would give Harry an unfair advantage over people who have regular brooms. And in Harry Potter and the Chamber of Secrets, the Slytherins with their new Nimbus 2001s would have an unfair advantage over the Gryffindors (they did; they scored a lot more points than Gryffindor). What's the logic behind allowing different brooms to be used in a competitive sport?
Many sports allow personal variations on equipment, sneakers, bikes, clubs, bats, car racing, etc. Quidditch is no different. There is no unfair advantage as anyone could use the same broom. Additionally, Harry is in a varsity/school/kid team which often have more relaxed rules compared to professional level/adult leagues.
To wit, Harry's broom does not seem to give him a significant advantage compared to other seekers, as they are always keeping pace with him. He's shown to be exceptionally good at flying with any broom, the Nimbus just keeping up with his abilities.