MWG7 works on a logical ruleset where the proxy is essentially "open" when no rules are set. So, you need to define rules based on what categories you want to block, not what categories you want to allow (because everything is allowed unless there is a rule blocking it). If you make a rule that only blocks categories you do not want such as porn, then search engines will be allowed unless they are categorized as at least one other category that you are blocking.
As a result of this structure it is best practice to restrict allow type rules only to specific sites that you definitely don't want to interfere with for special reasons.
I hope that helps.