They don't want job-seekers cherry picking or getting confused between full-time jobs and contract jobs (You can sometimes get a higher pay-rate contracting than as a full-time at the same company). I think, they also fear it reduces their recruiting brand. They want to make it appear that getting a job at X is really hard, which is why they boast about their low acceptance rate, etc. If people discovered there are hundred or thousands of contract jobs at the company, it reduces the allure. Use of contractors is also often not deemed as a 'good' workforce practice - similar to outsourcing, mainly because of stories of exploitation, etc. It also leads to complications- think customers buying some sophisticated or sensitive equipment or service from you discovering that you had a bunch of contractors building it.