It's probably Mr. Bennet is taking on a perspective of a spectator than the person responsible for the well-being of his youngest daughter, but Mr. Bennet clearly doesn't see that the buck needs to stop with him. The only person who makes any objection to Lydia going with the Foresters to Brighton is Lydia's second-oldest sister. Jane, Mrs. Bennet, Mary, neighbors, etc., don't say a thing. One reason Mr. Bennet doesn't object is because he's hoping it will actually do Lydia good - she will see her own insignificance. Sure, it's a justification on Mr. Bennet's, but if the only person who is seriously objecting to it is your 20-year daughter, does that really seem so serious an opposition (especially when your wife is promoting it) to an invitation from the Colonel in charge of an entire regiment? I tend to favor Adrian's arguments on this, too.