The problem arose for several perceived reasons:-
1 Garden hoses need to be able to continually flex and they need to have chemical plasticisers embedded in the materials to keep the flexible.
2 The grades of plastic used are often slightly porous to enable the placticiers to work, but this can also mean the plasticisers will progressively leach out into the water that passes through These can be harmful if consumed. It also means unwanted bacteria or other agents can easily begin to lodge in the inner surface of the hose and take hold.
3 The plastics used in garden hoses are often recycled material, and may contain types of plastics contaminated with hazardous materials which can be dissolved or carried out by flowing water.
4 Some of the chemicals used to colour the hoses and fittings can contain materials like Cadmium (yellow) which is highly toxic. These can also be dissolved or carried out by flowing water.
Food grade hoses are guaranteed to not to contain dangerous materials and will be manufactured to potable water standards.
The hoses used inside a caravan will have standing water in them most of the time, this gives more time for agents to be leached into the water. They are also subjected to much wider temperature swings and assume an warmer average temperature which can encourage growth of some algae's and other unwanted agents. This is why internal pipework should be food grade.
The plastic pipes used by the water companies and put into buildings are nowhere near as flexible as garden hoses, and they are made to the higher potable water standards
Whilst using a graden type hose on a water barrel filling station is not advisable, but becasue of its short length and the fact water will not be left standing in the pipe, the risks of contamination mentioned above are very low.