Maybe they felt this particular tip was legitimate... So now the police are supposed to know when tips are and aren't legimate? That's what you're suggesting and it create a tremendous burden on personnel resources... If they felt the tip to be legitimate enough to stake out the store, they should have felt it legitimate enough to place an undercover officer in the store as well...
You're missing the point: in this particular case they might have felt it was legimate... But to suggest they post LE at every location where they have tips would be burdensome...