Sky Goose,
I built my hours over two years of instructing and during that time I worked with quite a few other instructors who were airline pilots full time and instructing on days off. To be honest it never entered my mind that these guys were taking jobs from low houred people like me.
What you have to realise is airline flying is very regimented and at times boring. After a while all you want to do is to get back to flying something like a PA28 or 206 whenever you can to give a bit of variety. I just don't see why you shouldn't do this. If you have a problem with this then it is 'your problem', you need to deal with it, don't try and guilt these guys into not doing something that they love doing.
Thats just my view anyway.