Imaro
Legend
Even though it is for more particular thematic reasons, armor and weapon restrictions exist for every class.
Why does the rogue get limited to leather?
Why does the ranger get limited to hide?
Why does the Slayer get scale and the Knight plate?
This hasn't been true since at least AD&D 2e... there's no restriction, they start with these armor and weapons and can spend resources to expand them... and they certainly don't loose anything for using a weapon or armor not granted by their class.
So the question is, for the behavior limitations, in what ways are we giving Paladins to compensate? In 1e(from which all this is a holdover) you got better powers than other classes. As it stands, paladins have a limitation, but no corresponding enhancement to balance this out. This is one reason the alignment restriction on Paladins now seems so awkward, because Paladins are receiving no mechanical compensation.
Apples and Oranges... What compensation does a fighter get for protecting his fellow adventurers? What compensation does a rogue get for testing out trapped items and places? What compensation does a ranger get for scouting ahead? None whatsoever but it's (usually) how someone playing the class is expected to behave and if they don't behave in these archetypical manners either the party or the character is made less effective for it.