Are there many cases where the content of a preamble has been decisive? Or are the preambles mostly for show?
tl;dr: Statutory preambles are typically non-binding, and some states have laws to this effect (e.g. Florida, Illinois, Iowa, etc.).
The most consistent interpretation I've found is that when the body text isn't clear, the preamble might be helpful in determining context and legislative intent.
For example, in Shea v. Clinton, 850 F. Supp. 2d 153 (D.C. 2012), the district court said that where statutory text is ambiguous, courts may look to the preamble and legislative history for clarification.
This extends beyond the realm of statutes. In Catalina Marketing v. Coolsavings.com, Inc., 289 F.3d 801 (Fed. Cir. 2002), the court found that considering limitations in the preamble of a patent should be context-specific.
There is the case of the preamble of the constitution of the French 5th Republic. The constitution was adopted in 1958 and, until 1971, the Conseil constitutionnel ("CC" hereafter; more or less the equivalent of the Supreme Court, and rather less than more as it is not really a court) had only a minor role: it was a judge of national elections and referendums, and it also had to decide what were the respective domains of the parliament and of the the executive. Initially, the preamble was not thought to be binding, but in 1971, the CC invoked it to strike down a parliamentary law.
Actually, the preamble is rather empty but it refers to the following texts:
- the Revolutionary (1789) "Déclaration des droits de l'Homme et du citoyen", which states individual and political rights like those in the US Bill of Rights;
- the post-2nd World War (1946) preamble of the constitution of the 4th Republic, which proclaims some collective, social and economical rights. The latter itself mentions the existence of "fundamental principles recognized by the [ordinary] laws of the Republic", but does not provide a list of them.
The Conseil constitutionnel decided that the freedom of association was one these fundamental principles, and it has discovered (invented?) many others since. To be recognized as constitutionally binding, a principle has to appear in a law voted under the 1st, 2nd or 3rd Republic; it must also be stable and general.
It is on this fragile basis and through a very indirect reasoning to vague principles that the CC stroke down the law of 1971. Their power has however been validated and reinforced afterwards:
- since 1974, a small minority of the members of Parliament can ask the CC to examine whether a law is constitutional, just after it has been adopted and before it is promulgated (a priori; initially, only the president of the Republic, the prime minister and the presidents of the Assemblée nationale and of the Sénat could do this);
- and since 2008, courts may request the CC, after some filtering by the top courts (the Cour de cassation and the Conseil d'État), to do the same, even years after a law has been adopted by the parliament, so a posteriori.
A reference in English: https://en.wikipedia.org/wiki/Constitutional_Council_%28France%29#History_and_evolution