It's a restraint of trade issue. You're not allowed to restrain other's people's ability to run a business or earn an income, beyond some reasonable cases.
Like running the only gas station in town and then refusing to sell fuel to a competitor who is trying to build a gas station that wants to compete with you.
That is not analogous, as online travel agencies are just middlemen. I am not aware of a law anywhere requiring a seller to have to deal with middlemen.
Like running the only gas station in town and then refusing to sell fuel to a competitor who is trying to build a gas station that wants to compete with you.