Normally, if an aura is attached illegally to an object, it goes directly to the graveyard. It doesn’t pass go. It doesn’t unattach. And it definitely doesn’t collect $200.
A Bestow aura only stops being an aura and becomes a creature if (a) its target isn’t legal at resolution; or (b) it becomes unattached.
So this left a bit of a question about “What about when the aura is illegally attached due to protection?” Well, luckily, we have received a clear answer (aside from the one that Matt Tabak already gave us before the final Comp Rules update was released). They have addressed this issue in the most practical possible way. Just say “This is how it works!”
The answer, by the way, is that the aura just drops off and turns into a creature.
Here is the supporting rule that was finalized after the first draft of the Bestow rules was published:
702.102e If an Aura with bestow is attached to an illegal object or player, it becomes unattached. This is an exception to rule 704.5n.
Today’s Rules Tip written by Josh Stansfield