I wouldn't consider it trespassing, as the church is a public place (and open at the time they are doing this). If the church doesn't want it to happen, they should post signs stating "no solicition or distribution of fliers".
Reading some of the other posters, I agree, it's not really that big of an issue, just throw it away later (don't toss it out on the parking lot).
That said, eveyone is entitled to rant now and again, feel better now? lol