It’s fair because otherwise you could compensate people for zero tax. For example, you have a CEO that can either be paid:
1) $1m in cash
2) $1m in stock
3) an option grant to buy 1m shares at $0.000001. Each share has a FMV of $1.
Without AMT, you could always take (3) and they would get $1m of stock for $1. Tax free.