Seems perfectly moral to me. It's conditions attached to the sale of a business and I would assume such clauses are pretty standard. If I sell a consulting business to you, you'd be pretty crazy not to have some anti-solicitation clause in the sale agreement.

If I did open up a competing restaurant next door as soon as the ink was dry on the sale agreement, I think it would be on you and your lawyer not to have covered that eventuality. (Though it would also be kind of sleazy of me IMO.)