The problem is that butter knives have two common design features that make them virtually unusable with the left hand—the blade is asymmetrical and for some inexplicable reason, is offset from the handle. To use such a knife with the left hand, one must cut and lift a pat of butter on the back of the blade. Eliminating either one of these two design elements, losing the offset or making the blade symmetrical, would make a butter knife usable with either hand. I can't tell you how many times I've wanted to heat a butter knife up with a propane torch and flatten it out in a vice.

Perhaps most interesting of all is the fact that these twisted handle butter knives are not a new innovation but actually an older design that appears to have come and gone long before accommodating left-handers was given much thought. It seems the twisted handle was introduced in the mid-1800s and fell out of favor by 1920, a time in history when left-handers were actively discouraged from being, well, left-handed. I have to wonder if mass production techniques had a hand in that shift.
The use of elaborate, formal settings of flatware with numerous different specialized utensils such as consommé spoons and oyster and pickle forks, came into fashion with the genteel sensibilities of Victorian era. These formal sets of flatware included both a master butter knife used to take a portion of butter and place it on one's bread plate as well as smaller individual butter spreaders at each place setting that would be used to butter one's bread. The logic of this arrangement was that it would avoid contamination of the communal butter. The twisted handle butter knives would fall into the 'master' category, as would the offset butter knives we know today.
Like most Americans in the 21st century, I buy my butter in tubs and spread it with a table knife but I did acquire the twisted-handle butter knife pictured above and when I first tried it on a stick of butter I found that it took a little bit of getting used to but it was in no way awkward--or at least significantly less awkward than trying to use a conventional right-handed butter knife in my left hand. Interestingly, when a right-handed friend tried my new butter knife, she found it awkward to use in either hand. Be that as it may, I'm glad to have a butter knife that I can actually use.