Adding IPS browser exception signatures - cannot enable block

Why when you try to add an IPS browser exception signature it displays that it cannot be changed and by default is set to "Allow" and "log". Why

can I add the IPS signature but not change the setting to "Block" - what then is the purpose of adding signatures that will not take any action and yet it is recommened to be added ..

https://www-secure.symantec.com/connect/blogs/sundown-exploit-kit-adds-internet-explorer-exploit-any-other-kit

Case in point: IPS signature ID: 70103

• Web Attack: MSIE XMLDOM ActiveX CVE-2013-7331