meh youre half right.
the "bullet button" mag release my bro uses on his tinker toy was purchased separately, as you describe.
but his "bullet button" mag release tool snaps in and sticks to his button by the magnet, leaving a protruding removable mag release which can be taken out at the approach of lawer infursemint offisurs. he always referred to THAT device as his "bullet button" so i had to go get clarification.
interestingly, since most AR's are made of aluminium, your assertion as to the actual purpose of the magnet on the mag release "tool" is mistaken. it DOES stick to the recessed button, keeping it in place during rifle operation.
he further informs me that the lawer infursement offisurs at his favorite shooting range tell him his "tool" (a rod with a magnet on the end) is "Illegal" but the recessed magazine catch is not. (???)
according to my bro, the cop told him "you gotta use a bullet or it's illegal to use the bullet button", but from that statement either the cop at question is a moron, or my bro has poor english comprehension skills. either possibility is on the table...