...this is intended behaviour :-(
Placing any disk encrpyted machine into sleep/standby is generallay a bad idea, as the decryption keys used to access the disk remain in the memory, and may be retrieved should an unscrupulous individual gain physical access to the machine.
Best practice dictates that any disk encrypted machine is either entirely shutdown, or hibernated, and should remain in the user's presence for a few minutes after the power has cut out, to allow time for the keys to clear from memory. The good news is you are using SEE which can employ protection from cold boot attacks (http://www.symantec.com/docs/TECH96464).
The way I see it, is that the Pre-Boot Authentication screen is there to protect your encryption keys and therefore access to your hard disks. If the keys are already loaded into memory (as in sleep mode), then it adds no additional protection (plus you're not really booting the machine anymore).