No trouble at all getting what I want. It's not for anyone at the feed store to question me as to what I am going to use that bottle of Valbazen or Safeguard for and honestly, around here anyway, I can't imagine them even raising an eyebrow if I did say it was for my chickens.
However, I can understand employee's not being allowed to make suggestions to someone regarding the off label use of any product. Imagine if they were wrong, told you a wrong dose, etc. etc. Has nothing to do with you needing anyone's permission to use that product on your chickens, that is your choice. It's about store liability. So, you talk to other people who have been-there-done-that to see what works, or talk to a vet about products and doses, if you can find one who knows about chickens. Dewormers are not rocket science and most of them have a very wide margine of safety while being very effective, it just takes a little research at first to know what to use and when. Nothing to stress about!