Because leaflets like those were dropped over nearly every war zone in WW2. It was one of the simplest forms of propaganda together with radio stations. Russians dropped them over German forces telling them about war crimes and the likes. Germans dropped the same on Russian forces. British created radio stations that broadcasted in German and Germans did the same vice versa.

See http://en.wikipedia.org/wiki/Airborne_leaflet_propaganda#World_War_II for a lot of information.

Of course it was forbidden to pick up such a leaflet or even distribute it. That would imediately mean you are arrested as a spy and because of undermining the country's war effort. But it was not like most people believed what was written on leaflets anyway. People generally believed that the war was a just cause and when the enemy wrote "lies" on leaflets that only displayed how unhonorable they are fighting. Of course listening to enemy broadcasts was also forbidden. But those cases were a bit more tricky because the radio stations sounded legit to normal people. They used native speakers and claimed to be situated somewhere in the enemy's country or a neutral one. When they reported on news they didn't simply spread their propaganda, that would have been too obvious. They sometimes reported the same as official radio stations but altered the reports just a bit. So that the situation looks a bit more pessimistic, sometimes actually being closer to what really happened.