Because they're flying the A320 family rather than 737s. "Sharklet" is just the marketing name which Airbus is using for what is really their version of a blended winglet. There appears to be a subtle difference at the tip from the images I've seen, but other than that nothing different which would justify a new name.
Edit: By way of clarification, "sharklets" are the new winglets which are due to enter service in the next couple of years on the A320 family. The devices on the tips of current A320 family aricraft and the A380 are called fences.
No. Winglets/sharklets are good design fixes when you have certain constraints (span restriction, retrofit capability, etc.). But there will always be a class of aircraft where they don't make sense and you'll see raked wingtips there.