Well, Fantome is correct, but he must be an English teacher with a compulsive need to correct every grammar failing - or a lawyer specialising in precise definitions.
The wording is perfectly understandable to the vast majority of people, because our brains digest sentence wording and make sense of even jumbled and badly-misspelled words in sentences and paragraphs.
To satisfy Fantome, the precisely correct wording should be along the lines of ...
"Although amateur-built aircraft operated in the experimental category are not required to be fitted with a stall warning device, owner-pilots should consider the benefits of installing such devices (preferably with aural output) as a last line of defence against stalling."
The original paragraph construction places "preferably with aural output" in the same sentence structure that says, "not required to be fitted" - thus making the syntax incorrect.