Pack sizes were regulated in 1975 for volume measures (wine, beer, spirits, vinegar, oils, milk, water, and fruit juice) and in 1980 for weights (butter, cheese, salt, sugar, cereals [flour, pasta, rice, prepared cereals], dried fruits and vegetables, coffee, and a number of other things). In 2007, all of that was repealed - and member states were now forbidden from regulating pack sizes!
I think the rationale was that now the unit price (price per unit of measurement) was mandatory to display, consumers would still know which of two different packs on the same shelf was better value. But standard pack sizes don't just provide value-for-money comparisons, as this article shows.
Most butter here (and in a number of other countries) have measuring lines on the pack itself in 50g increments, so while I agree with you it's a nuisance to have an open one to deal with, the measurement part is usually a matter of using a knife along the marked line...
If the "certain brands" you refer to don't have those measuring lines, though, then a pox on them...
It's much easier finding unsalted butter in the US than in Portugal.
In fact, unsalted butter has been the default everywhere I've lived in the US.
Edit: not to mention, say, salted butter being a point of pride for Britany.