Does anyone know why the $Action.Activity merge fields generate an error message in the Force.com IDE when used in S-Controls?

I have the following line in one of my S-Controls:
this.location.href="{!URLFOR( $Action.Activity.LogCall)}&what_id="+oppid+"&00N60000001QCYa=Outbound Sales Call&retURL=/"+hhid

This works fine if you save it through the Salesforce.com S-Control window, but if you do a "Save to Server" from the Force.com IDE you get the following error:
Save error: Field $Action.Activity.LogCall does not exist. Check spelling.  

It appears that all of the $Action.Activity merge fields display this same behavior.  Other merge fields from the $Action.Account and $Action.Asset groups appear to work fine.