Not to defend the recommendation as they clearly have an interest in trying to get people to use as much as possible, there is a potentially valid reason for th

Not to defend the recommendation as they clearly have an interest in trying to get people to use as much as possible, there is a potentially valid reason for this, which is that it depends on the type of water you have. Water from a municipal supply is generally well-treated and usually soft, in which case you can get by using less detergent. However with hard water (often sourced from wells), more detergent is needed to get the same cleaning effect. Maybe they could try to explain this, or maybe they just ignore it for the sake of simplicity (of the instructions on the bottle), and the increased sales is just a nice side effect.