To be honest, a lot of it is probably the stronger employment legislation in the UK (and AFAIK the rest of Europe) as compared to that in the US. For instance, an employee sacked for refusing work outside his/her original contractual obligations can fight and win an unfair dismissal case. And in the particular case of working hours and being on-call, in most circumstances the employer has committed a criminal offence if they require more than 48 hours' work in a working week.

Peter