I don't know the exact laws about it, but the cops apparently get in trouble even if asked for doing it. A few months ago, some of them pushed smoothly an old man who was stuck with his broken down electric wheelchair on the express road (don't ask me what he was doing there, but the stubborn old man didn't want to abandon the chair, refused to call a mechanic and asked to the cops to push him, so they did what they thought right to clear the traffic):
http://www.dailymotion.com/video/x36u9z0_une-voiture-de-police-pousse-un-fauteuil-roulant-sur-la-route_news

The old man's family didn't make a complaint: on the contrary, they supported the policemen when the police union decided to penalize them.
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(off-topic, but I like this one :
http://www.dailymotion.com/video/x16t3oz_la-voiture-de-police-poulailler_creation
For non Francophone users: cops are called poulets (chickens) in French slang)