It's because instead of smoke-curing the bacon the old-fashioned way so that it'll keep for ages*, the producers use a short-cut ,and instead have to inject it with brine to try to maintain its keeping qualities, (and, by strange co-incidence, this results in an increase in weight!). Try as Catso says, and explain your preferences to the butcher - he might be able to help. Otherwise the 'sweetcure' type bacon in supermarkets tends to have a less salty taste.
*(In the true story of 'The Fetterman Massacre' a reporter with the US Cavalry wrote home complaining that the sides of bacon were 18 months old, and that the previous winter mice had eaten tunnels through them. It was still palatable. Can't get that quality these days!).