Yes, I believe they are always on the menu (all 3 times we've eaten there). We just ate there about a week ago, and my dh and oldest ds REALLY enjoyed the crab legs.
Yes, they moved the crab legs back to every night sometime last year (they were gone for a while, brought back for a test, left again after the test, then brought back every night).
No, they are not extra. They are part of the regular buffet. I remember when they used to be extra but as PP said they "mainstreamed" it sometime last year. We had a very good meal there last October.
IMHO at $37.00 per adult for a buffet they better have something special like crab legs to make it worthwhile especially when there are no characters.