Because fares are based on what the markets will bear. Business travel will accept higher fares than leisure travel.

Most leisure travellers (FT excepted!) will have a Saturday night in their trip and won't pay the kind of prices businesses stump up.

While occasional business travellers may be happy to sacrifice a weekend for the pleasure of an extra day away somewhere, most travellers won't so if they have to go then businesses have little choice but to stump up the higher prices.

It's a fairly simple way to differentiate the two markets and price accordingly.