I can't say I'm too surprised. Waterloo is unusual in that they offer a Bachelors in Math in addition to the more standard B.Sc. and B.A degrees. Computer Science majors there graduate with a B.Math.
The BMath option is only required if you want to have a double major in CS and a different area of math. Otherwise the BCS is strictly more flexible: you have 5 fewer math courses and 5 more electives (which you may use to take math courses if you really want to). From my experience most people are graduating with BCS unless they really like math.