ALL permgen issues are fixed by upping the amount of RAM you let JAVA use in this regard (its a flag you add to your startup script). Have a look around Ive posted on this subject a few times myself. However if youre stating why cant FTB up it themselves in the script that comes with the packs then the answer is I dont know, theres probably a reason.
This is actually incorrect. RAM allocated and PermGen allocated are two separate and distinct settings and must be edited as such. Also, these settings very much depend on your PC specifications, so writing a generic script may actually adversely affect performance for some people. Although, granted, this is less of an issue with RAM\PermGen, the FTB Team don't know how much RAM everyone that could possibly download their packs has, what Java version etc.