More generally some departments, including CS but not only, get away with picking above average students in a given college and letting other departments get the below average students, through things like "weed out courses" or not teaching actually 101 courses but "assuming a lot of background familiarity with computers", to quote what you say.

Difficulty of any given program at the undergraduate level is completely dependent on university policy. Maybe at the research level, you truly need to be more intelligent / harder working / whatever to succeed in math than in art history, but at the college level, you could do for instance:

- cram all that is taught in current undergrad, MA, and PhD (including the thesis) for art history in 4 years

- let CS students graduate if they can write any working program in any language of their choice

Tada! Art history is now the hardest degree in the college where only the best students aren't weeded out out; and CS is the easiest degree there is.

My point being that CS departments assuming prior levels of CS unlike other departments is merely an instance of a larger pattern.