25% is not token, that's 1/4th of the course! most math classes i've taken homework has been from .5% to 5% of your grade!
if your homework is 1/4 of your grade in the class, that class is a "pass me" class. in some math classes the prof refuses to acknowledge that homework actually has a measurable effect on your grade, but he will use its presence to round up instead of down (i.e. 89.5 becomes a 90 instead of an 89)
There are some classes where homework is more than 1/4 of the grade. Maybe not good math classes in the US.
Cheating doesn't happen when there's virtually no incentive. I stand by my point that you can cheat in Math homework more easily than equivalent non-Math homework, if you want to.
If 400-level Math coures are really impossible to cheat on, but there are 400-level non-Math courses that are possible to cheat on, I'd just argue that this is because they aren't really equivalent.
A student gets 80% on a 25% weighted "token" homework component, 60% on a 25% "lab" component (due to copying large portions of someone else's report), and 30% on the 50% exam (there's always a few low apples). They pass with 50%.
The lecturer sees that the exam is too hard, and makes it easier the next year.