I don't think it has anything to do with recruiter's ignorance or them believing that the integrated route creates a better pilot.
It simply minimizes risk to airline. They're in a position to make demands like this so why shouldn't they. A student on an intergrated course will be constantly monitored and under the scrutiny of many more points of assessment with a single organisation and can be discouraged from completeing the course if they're not suitable (I know the more cynicical of you may say that an FTO is unlikely to turn down business and you may be right). However, an airline can easily contact Jerez, OAT or whoever and get a good idea of the calibre of the student and assertain whether they'll sail through a type rating and go on to be reliable pilot and an asset to the company.