This is not a feature... It's a limitation.
MobileSafari keeps website cache only in RAM, and therefore, once it runs out of RAM, it'll automatically destroy an entire page, forcing the refresh, disguising itself as an auto-refresh feature. You can only keep at most 3 pages open at any time... Even the iPhone4 with double the amount of RAM can only keep 5 pages.
You can try a third-party browser that has offline caching, and it'll remedy the situation. Or alternatively, use the swap file mod I posted in the jailbreak forums and it should work, too.
These third-party browsers have offline page caching as far as I know:
iCabMobile (also supports concurrent tab, which means you can browse websites while listening to music on youtube, this is my personal favourite)
Atomic Web Browser (many useful features, but the interface is a bit strange)
Mercury (cool browser that looks like a shrunk version of desktop Safari)
Source:
http://www.ipodtouchfans.com/forums/showthread.php?t=306343