Alerts: Keyword filtering is not available for BM elements

Keyword filtering is not available for BM elements, this only shows for elements
when the condition uses reporting columns.

Per sharon: The elements that you can select should not be effected by whether
the rule is referring to elements in the bid targets table or the warehouse.
First, the user doesn’t know or care what table each field is pulled from AND
there is nothing that says you can’t change a rule after you add elements.
There is no logic in preventing a user from alerting on only one bid target even
if the rule refers to the bid table anyway.