well it's of course your right to do whatever you like.
But personally , it's not a moral issue at all: for me, there's no "should" or "should not" about it; it's simply a question of what works.

I actually think that it is far more fair and congenial to reward someone for having the courtesy of buying a ticket in advance with a reduction in ticket price: that's a commitment to the success of the event.

I know at least one folk promoter that would consider cancelling an event if they felt they had sold worryingly few advance tickets. I also know that if I am promising a visiting musician from America a minimum of £250 I quite literally cannot afford to have too many unknown factors with regard to how out of pocket I might end up being.