I think perhaps it depends on the type of masters you're doing. My dept offers 2 master's courses - 1 taught and 1 research.
I am on the research masters which means that the course is 12 months long and I am expected to work on my research project full-time until September 30th 2007 (I actually have to work full-time on it if I have any hope of finishing by the end of the year), which means that if I were an international student, I don't think I would be allowed to work during the summer months.
However, the taught masters is only a 9-month course, which ends in June. The students will hand in their research projects, take their finals in June and will then be completely finished, therefore free to undertake full-time employment during the summer (at least, the UK students can - the international students may not be able to work. It would depend on how long their visas were valid for, the rules of the visa, etc.)