Growing up as a conservative christian (a.k.a. Jehovah's Witness), I was raised to believe prayer is a private matter between you and your god. If people want to pray on a particular day or sitting on the toilet, that's their business. But to be commissioned by the government? I could have sworn there was some statement about a separation of church and state at one point.... like with the founding of this country... some guy named Jefferson? Even Benjamin Franklin's suggestions for a prayer at the Congressional Convention was met with polite embarrassment and (according to his own notes), “except for three or four persons, [they] thought prayers unnecessary.”
The news is constantly referring to Muslim countries as being religious-based nations. Is this in contrast to the U.S? Well, clearly! It's not like we have God printed on our currency, or have federally recognized Christian holidays, or rules about what can be bought, sold, and consumed on Sundays (the Christian bible's God's day of rest), or have God in the pledge of allegiance to this country, or have public policies (like abortion or gay rights) be based on biblical teachings, or give public funds to private religious (but only Christian) schools, or pray to God at baseball games (the nation's official past-time), or ever give credit to God for war victories. And you can certainly be a non-christian and get elected to public office. Indeed, the U.S. is clearly the most secular nation on earth! So why not add a day of prayer?
Granted, the official decree is that the prayer can be addressed to any god of your choosing, but then what about all those folks who don't believe in God? What about the agnostics who might acknowledge the possibility of the divine, but aren't convinced enough to actually direct prayer towards it? Prayer is just another type of meditative activity instilled in ritual practice that is a part of every religion. You bow to Mecca five times a day, you pray to God at least once a day (of if in a sticky spot... like a foxhole), you walk around the Peace Pagoda three times in each direction; these are all just ways to encourage the individual to reflect. I wish more people did cerebrate on the world at large and their place in it. If people did this more more often, there might be less violence and more compassion for fellow man. But to have it mandated by the government in a specified form on a specified day isn't exactly what our (government defined) religion (and tax) escaping founding fathers had in mind.
I do pray for humanity. But generally it's a direct appeal.