My guess about being warned off would be connected to the dangers of breakage.
The club where I shoot had a longbow guy who built up a collection of longbows for use occasionally by club members wanting to find out about longbow archery. The bows were also used near the end of the beginners' courses by anyone who felt like having a go at longbow archery as opposed to the recurve they had done on the course. It took a lot of his time to make sure the different weight bows were given to the right strength beginner, and getting their draw length to match that of the bow. Without a careful eye being kept on who was shooting which particular bow, I feel the bow may have been in danger of failure or the beginner may have hurt themselves accidentally by heaving on something too weighty for them.
I could very well be wrong; there again, there could be some archers who mistakenly think as I have described.
I think Nancepance's suggestion of buying second hand bows could be a reason, too. Money could be spent on something that is already 90% dead.