Well, the gods DID recognize them as true heroes at the end of Olympus’s second visit in KH2. I thinks it’s only fair that they allow them on Olympus because of their heroism.
Like Perkilator said, they are recognized heroes and TBH they are friends and helped out the son of the great god Zeus so... yeah. Also in the film they said that only gods can LIVE in Olympus but they said nothing about the chance of entering there...
