The spring is the difference. A switchblade (and "assisted opening" is just an obvious circumvention of those being banned) can be easily carried concealed and is a very deadly weapon in fights. That's why they are outlawed - legislation has decided that their potential utility (takes a second less to open?) doesn't outweigh their general danger to the public.
Note that all kinds of easy to carry knifes are not illegal, like hunting knifes or swiss army knifes.
The spring neither makes it deadlier (such a blade is not particularly deadly) nor does it make it dramatically easier to conceal carry (either is small enough to fit easily in my pockets). Had I not lost the plastic sheath (considering they are kept in cabinets anyway, what is the point?) to my paring knife, either would safely fit in my pocket and be deployable fast enough for any attempted homicide. The difference between the two as a murder weapon is stupidly negligible, the advantage of the spring and folding action are only cumulative over long term normal use. Sheath get lost, and regular pocket knives become tiresome; both issues that I assume are not a problem on the rare occasion you find yourself in a knife fight in a Broadway play.
The idea of spring loaded folding knives being super enhanced murder tools is a fantasy in the minds of people who have watched too many movies from the 50s. I mean seriously, what do you picture? People squaring up for knife fights in dark alleys where every millisecond counts? Give me a break.
Note that all kinds of easy to carry knifes are not illegal, like hunting knifes or swiss army knifes.