It is the same with all food dates. They only apply to unopened packs. Once open, it's important to check freshness before eating. Some manufacturers give an indication of how many days opened packs will last.
Food is often still perfectly okay for a limited time after the 'Best Before' date. (Obviously not yoghurt, haha) but the 'Use By' date is best adhered to.
Yes....I've only seen those use by dates a few times. I think I might go back to the one serving size yogurts - that way nothing will spoil because it won't be opened. I was trying to be a good steward with waste.