I don't think it's the "reasonableness" of a restriction that determines whether it's price discrimination. After all, the ability to buy a last minute fare is just a form of convenience like not having to take a connection. The strictest definition of price discrimination is different prices for the identical good. But that's hard for firms to put into practice. It's often easier to sell two goods of slightly different quality or convenience and have consumers separate themselves by their willingness to pay for the extra quality/convenience.

Some people will take price discrimination to include price differentials for two goods that is not justified by cost differences between the goods. Other people view that as a form of non-uniform pricing and not literally price discrimination, although price discrimination is obviously a subset of non-uniform pricing.