The rebels are claiming they killed more than four hundred soldiers. This less specific first sentence is called a soft lede. We get the listener's attention before we provide them any essential information. In this informal style, contractions are encouraged. Reading the text of the news story aloud must sound conversational.
Contractions are essential to that illusion.
Most of your school's daily announcements are likely culprits. Call attention to the difficulty students may have understanding the bulletin read aloud over the public address system. We'll have more to say about daily announcements in the next lesson. Suffice to say broadcast style favors active voice like no other style of writing. The boy always kicks the ball.