"3. Magical girls may not use their abilities against nonmagical individuals. The exception is when a nonmagical individual poses a serious threat to a magical girl."
I've just had a thought... The girls are literally confined to watching a crime take place... Say watch someone get kidnapped... Call the authorities... or help out without using special abilities...
BUT with the last one... Helping out as a regular girl immediately puts themselves at great risk since they're currently "regular"... This would then enable invocation of the latter half of the rule, the exception...
"So does that mean that when someone get's kidnapped in front of me I'm not allowed to intervene with my ability... However if I then call out the kidnappers and provoke them by like going "Hey I saw that... I'm a witness now..." And when they go and "silence the witness" they now pose a serious threat to my being thus I am entitled to dispense magical wroth upon them?"
"Yes, however... Since your identity is not a secret at all... They will most likely know you're a magical girl and not bother to silence you, thus not posing any threat to your wellbeing and in so doing you're unable to take advantage of the exception to the rule... Also you can't really stop a murder like that, even if the murderer does try to silence you, the deed have been done and someone has been murdered..."
"That... is really fucking sucky..."
"Well you have to leave something for the police to do, otherwise they'd go out of business"
"You idiot, being a police officer is not a business, it's a profession. Even if they have nothing to do they'd still get payed by the government"