The problem is that lots of languages, Javascript included, don't have decimals. They have floats instead. And you should never do monetary calculations (that you care about) with floats due to the floating point problem:
- Understanding floating point problems
- How to deal with floating point number precision in JavaScript?
- Is floating point math broken?
- http://docs.sun.com/source/806-3568/ncg_goldberg.html
As such, returning the rates in cents is a good way to remind programmers of this and avoid the temptation of doing financial calculations in floats.