No, you're exactly right. Tableau simply provides a connector to the Google Analytics API so if you specify the data by day in the Tableau interface this gets translated into an API call that asks for the data by day. The gotcha, as you've quite rightly noticed, is that it is incorrect to simply sum these daily numbers to get a monthly figure.
I guess one way around it would be to do three extracts yourself - daily, weekly and monthly - and then use some magic with parameters to show the right measure based on a drop down? Not trivial though....
Thanks for the comment Mark.
I have started to export in monthly and weekly to provide the metrics to complete our calculations.
I am facing the same issue.
Were you able to find any sustainable solution to this issue?