Some people don't enjoy work and paying them more might get them to work on time or to work the whole day. Or you could just fire them and hire someone who has an understanding that they have agreed to do a job for a rate of pay.
Oh really? Alright I can play your game: those people have agreed to do a job that involved standard office hours. Travel means being asked to sit in a cramped aeroplane for many hours and give up their evenings and potentially weekends to do their job. By your own measure, work travel spreads outside normal work hours, so making it comfortable is hardly an unreasonable request. Just because an employee asks for something doesn't mean they should be fired over it.