Where do you see the requirement to buy a full-fare ticket? The infant in arms has to pay 10% of the fare at the time the infant is ticketed, plus any taxes.
Infants traveling between the U.S. and Canada only pay taxes on the ticket. Infants traveling without a seat to other international destinations are charged 10 percent of the adult fare at the time of infant ticketing (it is usually less expensive to purchase the infant ticket in advance). Infants traveling on an adult’s lap on front cabin rewards or upgrades must pay 10 percent of the front cabin fare in applicable markets.
As for the disabilities thing, it is really in the "Special Needs" section which they apparently named wrong. C'est la vie.