I believe it says that in the park guide, under "Rules and Regulations," that wording with offensive language, wording, or graphics are not permitted.
Either security/admissisons staff isn't doing a good job, or some people are wearing their "F--- Everyone" (or something like that) shirts under a sweatshirt, and taking the sweatshirt off so they can go scot free.
Somewhat related, I went to Cedar Point last year, and they refused to let a young lady in who had a bikini top on, and had her shorts way below her waist. Security came and enforced the admission's staff requests that she put on a regular shirt (non-offensive), and pull her pants up.
I don't want to sound ultra-conservative here, but while she is not at school or work, she is not at a bar or strip club either. We are talking amusement parks here, and most of their business is dependent on families. Last time I checked, PKI heavily labels and promotes itself as a family park.
As for the smoking issue, the no-smoking-in-line rule could be enforced and stressed more. CP has signs in their lines, especially for Top Thrill Dragster, saying that "smoking is cause for removal from line." PKI could do similiar. As I know, there is nothing prohibiting those under 18 from smoking in permitted areas; they should ban this behavior, but that may cause more controversy than security needs right now, being in a post-9/11 world.