A British standard exists for motorcycle tyre repairs (BSAU159f), which determines the strength of the repair and where it can and can’t be made in the tyre.
Being a British Standard, it is a recommendation only, and not the law.
BSAU159f states that no repairs are allowed in the sidewalls (these flex continuously and a repair is likely to fail), and the standard says repairs are not allowed on high speed Z-rated tyres.
It says a tyre must be inspected by an expert for secondary damage (a nail which punctures the tread might also damage the sidewall inside), run-flat damage (determined by a darkened or marbled appearance inside the sidewall), exposed cords and so on.
Michelin, however, is the only manufacturer that recommends that its Z rated tyres can be repaired in accordance with the previous BSAU159e.
Repairs can only be made in the centre of the tread area, to within 25% of each side of the centre line.
Only two repairs per tyre are allowed up to the J (62mph) speed rating, and one up to the V (over 130mph with 155mph maximum) speed rating. Up to the J rating, repairs no bigger than 6mm are allowed, and up to the V rating only up to 3mm is allowed.
All repairs must be carried out with a mushroom-type plug, which fits into the puncture hole from the inside and bonds with the tyre by vulcanisation.
Anything else and the BS says the tyre must be chucked. If your tyre repairer says the same and quotes similar reasons, he’s being honest.