Ruphus: On the monthly plans page, there's a section for "Important things to know" which has "Terms and conditions and standard rates", then another link to the standard rates. IMO, not really hidden.
This. It's only one click from the signup page, not hidden, and links to a pretty easy to read table of charges.
To me it's more a case of not being on the correct plan for your needs, or the phone hasn't been correctly set up to only use WiFi for data hungry apps, which really needs to be done if you only have 3 GB/month.
I agree the casual rates are pretty steep when compared to any sort of plan, but they're not hidden or difficult to understand. A rollover plan is expected to be used by someone who won't generally get near the data cap (i.e. a low user), hence leaving a bit for the next month so they wouldn't normally get into the casual rates. Blowing through the entire month in a half day comes down to the user, particualty when Spark has sent text messages as the limit has been approaching.