Normally LUN shrink is a rare operation and it makes sense to shrink a LUN when a large part of that LUN is unused over a period of time. Probably Snap Manager treats that LUN as a whole data file from application point of view; hence not allowing doing such operation.
Can you please share with us about the use case for LUN shrinking?
Shrinking -- only available with Windows 2008. This will let you actually shrink the size of the lun
Reclaim -- available with DOT 7.2.1+, will let you free up FAS blocks based on NTFS block usage. So if you wrote a 50 GB file in NTFS, then deleted it, you could reclaim the 50GB that ONTAP used within the lun.