RY – when a Frum family adopts a goyish baby, they convert him as a baby and bring him up to be a Frum Jew the same way they do all their other children. Even though they know he can choose to be a goy when he turns Bar Mitzvah, they raise him as a Jew and assume that he will not choose to be a goy.
The same way that they assume that their other kids will not choose to become frei. Of course one is allowed and one is not, but that is not the point. The point is that they are assuming that a kid who is brought up Frum would not dream of giving up, whether he is allowed to or not. And if one understands what being Frum is about, that is a reasonable assumption.
I would hope that most Frum people are not Frum because they feel they have to be, but rather because they want to be. If there are any who don’t, then there is a problem either with the way they were taught about Yiddishkeit or the way they understand it.