Up to some value of volumeDelta * productCount it actually makes sense to use the same container due to sourcing/manufacturing cost from the variance so I wouldn't say purely disingenuous, there is real reasoning behind some of it... it's just marketing gets in the way of when you should actually add that second container size i.e. it's costing you more to always use the bigger container but the marketing department says the increase in sales due to confusion outweighs it.
Because demand curves don't work like that unless you have a highly inelastic good. With normal goods keeping quantity constant as price changes puts you on a suboptimal point on the demand curve. Coffee's price certainly hasn't been inelastic and changing can size at every spike would have netted you decent production cost increases instead of savings: http://www.aboutinflation.com/_/rsrc/1368022029715/coffee-vs...
The varying of quantities in the same sized container is not the problem and is certainly not illogical. The problem is when we allow marketing departments to lie/cheat/fool the product onto a different portion of the demand curve instead of letting the $/amount naturally flow back when price decreases or when it makes sense to switch to cheaper containers.
Because it pays, and for some reason not many people have the guts to say marketing is rotten to the core, and advertising is cancer on the society. Maybe because the market encourages it structurally. Lying costs little and yields good profits, so acting immorally gives competitive advantage.
