trux:
Actually, I think you will find Hato are NOT allowed/supposed to do that for general public either !! 
For the same reason Bravo G C mentioned and possible litigation + proportion to blame if something goes wrong or pear shaped (Blame the Americans - I`ll sue your zb s boy – for starting it ,)
ie :- wheel falls off later/wheel studs snap/ threads get stripped on nuts-studs / no torque wrench / incorrectly fitted , whatever ,
Not their role - not trained - not qualified to change wheels , and all for reasons of health + safety. simple as , apparently.
Each man /woman to their own job role.
You wouldnt ask a full time eg:- window cleaner, who has never had wheel tyre fitting training, to come and change your trailers tyres would you 
Hato role is to provide Safety advice - Scene Protection if needed/requested - guidance if suitable - and assistance in aiding a safe resolution i believe is the criteria . So I hear.
Unless of course , someone knows different . 
IT’s true, there are small risks associated with the fitment of wheels, but you really do have to be a complete bellend to do anything like strip the threads/snap the studs or put the wheel on “incorrectly fitted” (what, inside out?). If it was that dangerous, why would manufacturers give the general public a spare wheel, jack, brace and instructions in the cars owners manual to how to go about changing a wheel in order to get them on their way to the nearest tyre garage?
I’d be very concerned if the bloke in the big flashy light vehicle who’s able to create rolling road blocks, tow people to safety and sit on the hard shoulder to protect stricken vehicles didn’t have enough common sense to change a wheel on a HATO vehicle (car). It’s probably more dangerous to sit on the hard shoulder for an hour or more to wait for recovery, rather than pull as far left as possible and get the wheel changed in 10 minutes and be off. Undoubtedly, there are certain situations where changing a wheel would be too risky, depending on where the vehicles located for example. And of course, to cover your ■■■ you should get it checked immediately by a “trained mechanic” (the same numpties who I’ve seen using wrecking bars to tighten wheel nuts ) to make sure you haven’t got the wheel inside out or something along those lines. Of course, I’m fairly certain you HATO’s are people with common sense, the ability to read simple wheel-changing instructions if you need to and posess adequate authority over your hands to be able to carry out the job. It’s just a shame the powers above you don’t think of you in the same way!
It’s all about blame and liability of course, and risks blown out of all proportion.
Trust me, we fight against the namby-pamby ways on a daily basis. I’ve just had a weekend full of them! 