In iCal 5 you can drag the accounts in iCal's calendars list. Open the calendar list, click and hold on the word GOOGLE and drag it below the iCloud calendars. I can't test to see if this will change the behaviour you are seeing.
The default calendar as far as Mail is concerned will probably be the first one in the list. I agree with you that it would make sense to link it to the one set in iCal, I'm guessing this is a hangover from when iCal did not have this preference.
I have successfully fixed this problem for myself by doing the following:
- Set my default calendar on my iOS devices (synced via iCloud),
- set my default calendar within iCal's preference settings,
- dragged my default calendar to the top of the list that pops up when pushing the "Calendars" button on the top-left of the main iCal UI, and, most importantly,
- by trigging an add to my calendars from a Mail message and pressing the "Edit" button, then choosing the target (default) calendar I am interested in.
This final step seems to be the thing that "sticks" for Mail's support for iCal integration, as all future implicit calendar entry additions now target the newly selected calendar.