because the cost of writing the manual is a sunk cost. The money they charge for a manual should be postage and paper since the products they sell are electronics. They are not a publishing company. So if you download a manual, they do not pay for paper or postage. Therefore it should cost you no money.
It's sellable "product" because there's a used market that needs it.... the fact that it's a by-product of other products they sell really doesn't enter into it..........
Once again, fair business practice....
This is not really different than a recording studio charging separately for recording media.... the argument "...you're not in the recording media production business should you shouldn't be charging for it..." makes no sense........!