It's an invitation to take a seat, not take a chair. The rockers are protected by steel cables running through them, so one may sit in them but not take them away. It makes sense on a college campus. Otherwise they would be sure to add luxury and comfort to dorm rooms somewhere. Yet, it still is a mute comment on society, trust, and the English language. There's a discomfit that we must fasten down anything of value lest it be lost. And pity the poor student of English as a Second Language who tries to make sense of being told "You may take a seat but not the chair."