This graph was generated by one of the FF devs in testing to see which of those browsers cleans up it's memory well. You can clearly see that IE7 does not clear anything, and just continually eats up more memory.
Tell me, how exactly do you know what's going on? Did you see the source? Analyze it yourself with a memory viewer? No, you saw a graph representing the memory usage of a browser and drew your own wild, totally off-base conclusions.
The problem is not that IE7 is "keeping web pages in RAM," it's because of the way IE handles multiple browser objects. If IE kept all pages in RAM, previous versions would have killed the very systems it was supposed to run on very quickly (when I first started using Windows 2000 back when it came out I used IE5 as my daily browser on a P2-400 with 128MB of RAM), not to mention the memory usage would build up a lot faster than it does.
When it comes to very basic usage, IE has always been easier on memory than Firefox without exception (and with Firefox 3 and IE8 that's still the case, only there isn't AS MUCH of a difference now -- only around 10MB). The only reason Firefox is better at it as a whole is because it's always managed tabs and windows better. The main "issue" with IE is the way it handles new sessions of the browser -- it creates new IE controls with every tab/window that gets opened if I remember my Windows-ese correctly. The issue isn't that the browser is keeping pages in memory, it's that it doesn't clean up immediately after it releases the controls.
The numbers fluctuate far more than the blog post shows, too. I put heavy load on my system (which took a lot of work -- namely, Crysis x64 and 12 instances of EVE Online, which took up a total of about 3.2GB of memory...my system took it surprisingly well, with Aero only dropping to about 5 fps or so) and, surprise, it started garbage collecting like most built-in Windows components quickly do when memory is low (and, after that, I performed the surprisingly quick task of closing everything I'd opened and my system thanked me).
Given enough time, most Windows applications do release the memory that closed controls use. It generally does this when the application is in the background for a while.