My guess is that there was a mixture of: 1) This not being a priority for FB 2) FB Eng / PM being too clever for its own good and not doing the obvious thing of substring matching ("not scalable"). (Arguably a part of (1) - if it's really important, you'd have the team to maintain a list of banned strings). 3) Just a plain old bug (although arguably this is a subcategory of (1)).