It's common rule of Russian. "з" sounds like "с" in the end of the word. If the word is followed by other word beginning with a vowel or a voice consonant, and there is no pause between them, then "з" keeps its voice.
Examples: через день [cherezden'], через час [chereschas], без обмана [bezobmana], без причин [besprichin].
NOTE: беспричинно (adverb), чересполосица (noun).