The NSW GOVERNMENT permits the sale of mobilty devices for those with disabilities that can be classified, by the NSW GOVERNMENT as non compliant. Example mobility scooters come in many sizes. The large ones offer many benefits but according to the NSW government are non compliant with their standards especially public transport.
I know this may not make sense but I have this in writing from the now NSW treasurer from when she was trnsport minister.

What right has anyone to dictate what others may or may not purchase?

If certain devices pose a danger to the public, then don't allow them on public roads, but that doesn't give you permission to stop people from buying whatever they like so long as it stays on their own premises (or the premises of others who agree to it).

I don't know which devices are specifically being referred
to here and what the problems are. But, I would have
thought that safety would be a primary concern in what is
or is not acceptable both in regards to the person with the
disability and others. I imagine that there are standards
that regulate mobility devices that have to be met.
And it makes sense to have these standards.

Lamp, obviously some of the very large gophers are too big to go on public transport, as they would take up too much room and be a danger to feet all around them!
I have had a small gopher run over my well-shod nursing boot and I had a huge bruise.

Quite often, some of the disabled males, who were the sort who clung to their car long after it was safe for them to drive it, bought big gophers that they didn't really need or were too big for them to handle safely.

These sort of gophers aren't suitable for public transport vehicles. and should only be used for personal use to get them from home to shop on the footpaths etc.
The Government doesn't need to ban them outright, but they do need to have rules re their use.

Suse,

Nice to the point post; non compliance but approved for use has absolutely nothing to do with safety but, as you say, has everything to do with size, some of these devices are almost in the 'light truck' category.
They won't even fit into a public transport 'bus.

I am all in favour of mobility devices, believing some of the 'Usual Suspects' on the forum who presently rely on their 'Zimmer Frames' to get them out and about from the "home" will find them a god send, particularly for the fortnightly bingo outing. It gets them away from the homes PC and posting forum dribble, and out in the fresh air.
BUT, fellas could you please restrict your speed to 5km/hr in built up areas, like shopping centers, and on the open road, try to keep under a 100 if you can.