Seems someone doesn't understand.

Let me put this simple
tiny things can pass bigger things
bigger things can't pass things that are strong in numbers(seeing as the reiatsu in the petals ARE strong enough they can block off any other oncoming reiatsu/reiatsu infected object)
And no a human can pass through those petals(aren't real petals but are made from reiatsu) and reiatsu isn't a solid object so other objects should be able to pass through it if the reiatsu isn't strong enough to kill you on contact.
Hope that helps you understand more