I always release them and yes, I do see how they should not attach themselves and I am a bit guilty for letting it happen
but then again I usually notice an effect where at a certain age (puberty ?) their behavior radically changes and one will know they want to fly, the sooner the better. It also appears they can tell humans apart and only a few days after release, when you come across them again, they seem a bit less shy than the usual wild bird but wont let you get too close either. I think that should keep them safe.