If the weather is foul then it's possible to get quite wind blown or soaked even on a 10 minute walk. There may also be times at night when a girl may not be comfortable walking alone, particularly if new to the area. Easier to just have a consistent policy to avoid punters complaining about being charged this time when last time not.
Complaining about this is a bit like complaining a girl charged you extra for cim, but she only spent less than a minute with cum actually in her mouth. Or complaining about being charged outcall rate although the girl didn't have far to travel to get to you. If you want a chargeable service, then you pay for it. So as long as it is stated on the website that duos incur a taxi fee if the girls aren't at the same address, I don't see the problem. The OP seems to be expecting a freebie.