The injection method (which is where it shows up on the page) comes into question sometimes PERIOD across the net with many providers or anything else... its in one of those grey areas... its not illegal.. but should it be done? (means the ISP is slightly modifying the packet being sent to you to show that).
The reasoning WHY its done that way? (as far as my understanding)
There are many people who have an email set up with their rogers when they first got it... and NEVER use it. This is where the usage notifications go. There are people like my inlaws.. who are on the internet like every day.. but only check their emails every few days.
These sorts of things... if via email.. the user might NEVER see them.
Doing it this way, while intrusive... ensures that SOMEONE will see it come up, no matter what they are doing, so they are aware of their usage levels. Click the confirm, you should not see it again (till the 100% warning)
As mentioned, you can turn off the notification alltogether i beleive... but like they said, you have to monitor your usage 100% on your own then.
(which you can do via the myrogers page, or even the mobile app will show it now)