"Formal admission into the OMS CS program will require a Bachelor of Science degree in computer science from an accredited institution, or a related Bachelor of Science degree with a possible need to take and pass remedial courses." -- http://www.omscs.gatech.edu/faq/

In my experience, at least with average state schools, their MS in CS curriculum is--to a degree--a rehash of their undergraduate curriculum. Often times they'll just assign more work, or slightly harder work. I'm sure it depends on the program, but many programs seem to assume that even if a candidate comes into the program with a BS in CS, they'll have to have their memory "refreshed" to a significant degree.

Also, there is an element of selection bias. Those who are likely to apply to an MS in Computer Science self-select according to their perception of the requirements of those programs. This enables programs to capture those without a Bachelor's degree in Computer Science but who still have the level of motivation, acumen, and interest necessary to succeed in an MS program in Computer Science.

Having never done an MS in CS...

So long as they know the fundamentals of programming (through, say, the equivalent a Data Structures class) I think a mathematician could take a go at it without much issue. An EE probably wouldn't have too much trouble either.