Unfortunately, this has happened to some extent with CS programs offered through the math department. CS is so severely impacted at a lot of UCs, and a lot of students (probably pretty good ones) are denied access to the major, so some students look into doing an alternate CS-style major through the math department. The reason I say "unfortunately" is that I think the students who get bounced from CS but make it through math could have made it through CS if there had been more resources for them. It's stupid to have people who really want to be majoring in CS working through Real Analysis proofs that they don't enjoy or even particularly care much about.
Some of this is just because math is a bit cheaper on the budget. A whiteboard and a room is all you need for most math classes, so they tend to take most people who want to give it a crack. It's pretty hard to get into an upper division CS class as a non-major, because they barely have space for the majors (again, I don't think this is the case at well funded private universities with small undergrad populations like Stanford or MIT - it's more of a problem at Berkeley and other large state-supported institutions). Maybe it's also cultural within the field. If you go up to a math professor and ask if you can join his class without the pre-reqs, he's more likely than a CS professor to say "well, as long as you think you can handle it, sure." I suspect math professors are just a bit more laid back than the CS ones.
But overall, I do agree with you - nobody drops out of a "hard" major to take the "easier" classes in the math dept.
Some of this is just because math is a bit cheaper on the budget. A whiteboard and a room is all you need for most math classes, so they tend to take most people who want to give it a crack. It's pretty hard to get into an upper division CS class as a non-major, because they barely have space for the majors (again, I don't think this is the case at well funded private universities with small undergrad populations like Stanford or MIT - it's more of a problem at Berkeley and other large state-supported institutions). Maybe it's also cultural within the field. If you go up to a math professor and ask if you can join his class without the pre-reqs, he's more likely than a CS professor to say "well, as long as you think you can handle it, sure." I suspect math professors are just a bit more laid back than the CS ones.
But overall, I do agree with you - nobody drops out of a "hard" major to take the "easier" classes in the math dept.