in the usa, most state operated and private riding areas and parks require a spark arrestor or baffle.
the spark arrestor is for more dryer areas and areas with lots of low vegetation, keep in mind 1/4-1/3 of our nation is covered in tallgrass plains or crops for food/animal feed.
the sun and heat alone can start a brush/plains fire, so obviously a spark from a exhaust could start one.
yes the police and game/park wardens do inforce the arrestor/baffle laws, depends on the area most of the time.
and to the first few replies, thanks for being rude and ignorant about the op's question, instead of posting useful info.