They estimate a market segment.
They determine their UI.
They pay people to implement the desired UI for the desired market segment.
It's a question of numbers.
Does the remaining market segment that can't utilize their site justify ANY expense?
Well, lets see.
They are either too cheap or too poor to have the barely minimum acceptable gear, in order to view their site.
So, why would they spend a penny chasing an obviously smaller reward, when they could making their site prettier for the people who do have the extra cash lying around?
Bad for business. Actually, it is pretty good to come up with some limiting factor that keeps the riff-raff out, that way customer service time will be spent on customers more able to buy more.