At my Tesco on the pay pillar there is air and screen wash.
There is also a tap with water and it says "do not drink"
When you operate the tap it is obviously under mains pressure. My guess is that they put the "do not drink" sign on just to cover themselves. If it comes from the mains it should be OK. In any case I put purification tabs in mine as it may stand around a while before use.
Anyway, assumptions you make about mains water are obviously at your own risk, but often supermarkets put on these kind of warnings to cover themselves in these days of stupid litigation claims. A classic example would be "Recipe does not contain nuts but cannot guarantee nut free as equipment used to prepare food previously which may have contained nuts... yada yada"