I agree that it isn't right for you to be employed on a per diem basis and be forced to work five days a week, because if you wanted to do that you could just go find a part-time or full-time job. I have worked on a per diem basis for a couple of different companies and their policies are generally that you must work one summer holiday and one winter holiday, and then work a set number of hours per schedule, such as 48 hours in six weeks, with at least 16 of those hours being on the weekend, or something like that. If you are working five eight-hour days a week, then you are working 240 hours per six weeks, which is a far cry from per diem, IMHO!
I also agree that it is wrong to intimidate you by calling you and trying to force you to come in when you call off. That is just wrong, unless you called off every week or something. However, being employed on a per diem basis should not give you the leeway to call off more often or be treated more softly when you do call off, because the bottom line is, whether per diem, part-time, or full-time, when you agree to work a specific shift, the expectation is that you will be there, and you still leave a hole to fill in the schedule, regardless of whether you work per diem or otherwise. A sick call is a sick call, no matter who it is who calls in.