煙草 is ateji which in this case means that the characters are read that way only for that word. The other kind of ateji is when kanji are used like an alphabet

煙草 is ateji which in this case means that the characters are read that way only for that word. The other kind of ateji is when kanji are used like an alphabet eg 亜米利加 (A me ri ka). The second kind is mainly used for abbreviations in modern Japanese eg 米 is equivalent to writing "US" in English.