Same misunderstanding as Alan -- things are banned (or at least not encouraged) that might cause significant risk -- including death -- to other people, especially if done by non-experts. Like DIY firework mortar manufacture...
And before you or Alan or anybody else says "but I've been refilling bottles safely for ages, there haven't been any such explosions, you're scaremongering" -- today very few people do this (and probably only the ones who are knowledgeable and careful), I guess 99% or more buy the bottles from Calor because it's easy and safe.
If the sale of the smaller bottles is stopped and large numbers of people switch to DIY refilling them instead -- encouraged by posts in the internet (including here) telling them how cheap and simple it is -- then the risk of overfilling and a subsequent explosion will be hugely increased compared to today, and sooner or later somebody will die as a result.
Even though I'm sure this will get the same objections again, I'm going to say it again -- it is dangerous and irresponsible to encourage people to DIY refill gas cylinders (especially large ones, as opposed to the tiny Camping Gaz ones) when the only thing stopping it going horribly wrong is the care and expertise of the DIYer, which is absolutely not guaranteed to be up to the job for many people... 😞