Opt-in is quite obviously not the same as opt-out (via restrictions) so it's dubious to compare the two. The parent poster was asking for a way to allow iOS users a way to voluntarily and explicitly enable content that may be deemed objectionable by some. It's not the same as restrictions, because the device is passively not going to allow those types of applications.
It is effectively the same. Unless the parent sets up parental controls (which they aren't going to do) there's nothing stopping the kid from opting in.
You're making unfounded conclusions about a hypothetical feature Apple hasn't implemented.
I can think of at least one way to make it difficult for a kid to opt-in (I'm sure there are others): require the opt-in to be done online and require re-authorizing the credit card associated with the iTunes account. If it's good enough to get an email account out of a COPPA jail, it's good enough here.
