It's not just the employers that are taking advantage of this situation, its the employees as well.
We have a huge problem of under-employment in the UK with masses of working age people CHOOSING to work minimal part time hours in order to receive optimum "in work" benefits, even when permanent full-time hours are available to them.
I saw it first hand in my previous workplace, even though the pay was around 20% higher than National Minimum Wage there were still numerous staff members insistent on only working part-time even though permanent full-time contracts plus guaranteed overtime was available to them... all because they were better off working around 16 hours per week and receiving "top up" benefits than they would be working 40-50 hours per week.