EasyJet will not be breaking the law, just using it to their advantage. As part time or casual workers the holiday entitlement can be based on the number of hours actually worked and then " annualised" to provide a comparison for entitlement. In theory, if you worked 4 hrs a day for 3 days, thats 12 hrs, which is rolled up to be 1.5 working days. As holidays are based on a minimum number per year for full time workers and pro-rata for part time and agency workers, which can include Public Holidays, under these calculations you would have to work a lot of hours to get the basic entitlement.
Is it morally right? well that's a personal decision, but I guess most flexi crew will have over half the month off in any event..allbeit unpaid.