In fact zero-tolerance is a symptom of thoughless fanatacism. While "zero tolerance" itself might not explicitly contravene the Constitution, the principles behind it have certainly been disfavored in American law. In rejecting the concept of mandatory death penalties, the Supreme Court cited the case of Pennsylvania ex rel. Sullivan v. Ashe, 302 U.S. 51, which observed:
For the determination of sentences, justice generally requires consideration of more than the particular acts by which the crime was committed, and that there be taken into account the circumstances of the offense, together with the character and propensities of the offender.
Note that the Court is refering to what justice requires, and that it quite explicitly is "the circumstances of the offense." The whole point of "zero tolerance" enforcement is that the circumstances of the offense is irrelevant. We may then logically conclude that "zero tolerance" policies are contrary to what "justice generally requires." But that is not all that the Supreme Court had to say on the matter. The death penalty case that cited Ashe was Woodson v. North Carolina, 428 U.S. 280, which also contained the language:
While the prevailing practice of individualizing sentencing determinations generally reflects simply enlightened policy rather than a Constitutional imperative...
So we see that consideration of individual circumstances in assessing infractions is "enlightened policy" and that ignoring such considerations must be less so. Thus, the Supreme Court has opined that ignoring the individual circumstances of a rule infraction or transgression of the law conflicts with both justice and enlightened policy.
The phenomenon of zero tolerance policies reflects several not-very-appealing traits in the officials that support them. One such trait is obviously cowardice, hiding behind inflexible policy in order to be spared a controversial decision. But this is not the whole story. Many otherwise well-meaning officials devolve to "zero-tolerance" troglodytes. They may start off with good intentions, and adopt a particular cause which they then embrace with greater and greater fervor. They come to accept that what they believe is so right that it must be a virtue not only in specific cases, but a virtue in general, so that there can be no counter-examples to refute the righteousness perceived.
Zealots see zero tolerance not as a dictum of lazy management, as much as an expression of a universal truth. Of course, anyone who reasons this way has crossed the line that separates reason from fanaticism. The same malady of thought that perceives the sex-offender in the affection of a kindergartener is the same that senses the threat of racial impurity, or of ideological infidelity. Fanatacism is a disorder of believing that other people need to accept the fanatic's view of the world, and must live in accordance with that view. It is one person driven to hysteria by people living their own lives.