Straight from the source on Google Groups:
"On Wed, 29 Aug 2012, Brian Chesteen wrote:
> How to stop the "Most active areas" blocks from taking over the screen?
>
> Example:
>
> I have a filter applied to only show airplanes which works great at this zoom level
> but only shows a limited amount of the country:
Currently there's no way to block that.
They are there to prevent zooming out from killing (1) your browser, (2)
my servers.
Even though filtering helps against (1), it doesn't help much with (2) as
the server has to figure out which stations and path line points to pass
on to your browser, out of all those stations within the country, again
and again every few seconds as the map is updated in real time.
As hardware advances, and with some software optimisation, it might be
possible to make it allow zooming a bit more out, but not much.
* *- Hessu "