It was the Founder's intention that's why. The precepts of Biblical Morality, Charity and Law were fundamental for both the establishment of this Republic, and it's survival.
The fact that we have degenerated to the point that religion is offensive, is a harbringer of this nation's impending doom. Our Founders warned us of this.
Our Founders were men of differing denominations, sects and scriptural holdings in regards to Scripture and Christianity - but they ALL agreed and came to gether on the foundation of both God's Law in scripture and the precepts of Christianity as necessary for liberty to survive.
NEVER would they have been offended at a Baptist child passing out "Jesus Loves You" flyers to a bunch of Methodists and Anglicans in the town school. It would have been a springboard of agreement and even a possible lesson to be taught had the teachers seen it. Is it truly harmful to me and mine if some kid passes out pencils that state "Allah is God"? Heck no! I can utilize that as an opportunity to teach my own kid.
If YOU are offended by Christian religious expression - that is your own problem, the majority of Americans adhere to one form of Christianity or another as has been our entire history since the Mayflower, so why do you seek to infringe on our free exercize?
At one time in this Republic, such devotion was a duty - as even George Washington DEMANDED all his troops attend religious services if not on duty. Likewise, our Founders considered it a DUTY for the people to be a religious and holy, moral people for our nation to survive:
"No human society has ever been able to maintain both order and freedom, both cohesiveness and liberty apart from the moral precepts of the Christian Religion applied and accepted by all classes. Should our Republic ever forget this fundamental precept of governance, men are certain to shed their responsibilities for licenteousness and this great experiment will then surely be doomed" - John Jay