Distributors often require proof of purchase before any warranty work is begun. This is because items can often be purchased overseas for less than they can in the US, meaning that the distributor never dealt with the item and is then left holding the bag on the warranty.

I can think of a couple good examples of this. Back when the exchange rate between the US and UK was very favorable to the US, I purchased an Irvine 53 from the UK. Sig is the sole US distributor of Irvine engines. Had something gone wrong I'd have had to send my engine back to the UK for warranty work. Sig shouldn't be responsible for such issues. Futaba also had this problem with some of their gyros. Folks were buying them from Hong Kong at low prices. Hobbico is the US distributor, and if the goods didn't pass through them expecting them to do warranty work isn't reasonable. This is called a grey market.

Horizon's position on the issue isn't unreasonable, and is quite common in the hobby IMO.