It's not absurd. Just because it's available somewhere doesn't mean it's easy to find. Also you're having your arm twisted to purchase something that will probably be used very little elsewhere in your life. Why not just use the more common screw and make things a bit easier?!
It's almost like a company using an uncommon electrical plug on their power cable, and expecting everyone to buy an adapter to use it. People would be outraged.
If there is a valid mechanical/electrical/physical reason why they would choose one fixing over the other, then fair enough - sometimes the more esoteric product can be the correct or only one for the job. If they're just doing it for the sake of making it harder to work with, then, by definition, it's user hostile.
Those are Robertson-bit screws, and are ubiquitous here in Canada. (Also a Canadian invention, don'tcha know.)
https://www.robertsonscrew.com/
They work great with a power driver, such as you'd want for putting in a deck or fence, so 2-3" deck screws are like my version of duct tape around the house.
It's almost like a company using an uncommon electrical plug on their power cable, and expecting everyone to buy an adapter to use it. People would be outraged.