The Cures are told to conceal their identity despite the fact that the enemy has seen them transform numerous times.
They're keeping their identities secret from their families and friends (not enemies), so as to not worry them and to maintain private lives.
Indeed, in episode 42 of Smile Precure, Majorina forces Nao to morph into Cure March in front of her siblings.
In every even-numbered continuity of Precure (counting both Futari wa Pretty Cure seasons and both Yes! Pretty Cure 5 seasons as one continuity each), there has been a HeelFace Turn for at least one female character who either became a Cure herself, befriended the Precures so closely, or fought alongside the Precures as a Cure. The emphasis on "even" is fitting considering that those characters are the ones on the most even terms with the Precures: