A full party of mid/upper 20s can easily train at a location with level 40+ mobs, but for doing higher level and more difficult content you actually end up getting less exp than grinding easier content your level and below. It’s strange how I got better exp at mines than at higher level spots when I could 2-shot the mobs at the higher level spots. Removing the experience penalty from killing higher level mobs also encourages group play by getting people who would normally be too weak to fight the mobs to party together.
Also, since someone mentioned the possibility of high level players boosting low level players at high level areas in another topic: there should be no cutoff point for exp gain, if you have the connections, the gear, or the money to find a group of people to boost you, you should be rewarded. Some rich guy with connections or someone who saved up strong low level equipment being boosted does not have a negative impact on the rest of the playerbase who are leveling normally.