In the United States, federal courts are not allowed to issue advisory opinions under the "case or controversy" requirement specified in the Constitution. Many states also do not allow their courts to issue advisory opinions.
Why are advisory opinions prohibited? What's the rationale for the "case or controversy" requirement?
Advisory opinions violate the separation-of-powers doctrine. The "case or controversy" clause helps enforce this separation. The judicial branch is responsible for resolving legal disputes by interpreting and applying existing law. In doing so, it may incidentally modify or extend the law. Courts are only supposed to do this to the extent necessary to resolve disputes. Outside a legal dispute, the legislative branch is solely responsible for making changes to the law.
If courts were allowed to issue advisory opinions, they would be effectively be altering the law without addressing an actual dispute. As such, advisory opinions violate the separation-of-powers doctrine. The "case or controversy" requirement exists to ensure that the judicial branch does not usurp the powers of the legislative branch.
Relevant quote from page 45 (PDF page 10) of Separation of Powers: An Old Doctrine Triggers a New Crisis:
[...] one of the major arguments [...] against advisory opinions is that they violate the separation of powers doctrine. Judicial power, it is said, should be limited to deciding litigated cases. When justices issue opinions on contentions that have not yet been the subject of legal dispute, these justices approach the status of lawmakers.
The rationale for the case and controversy requirement is that first, you don't want to burden the courts with resolving hypothetical questions, second, that the law, particularly in common law systems descended from English law, is very fact and context sensitive and may not have a well determined answer in a vacuum, and third, it insures that the courts are not used collusively or by people who don't have an appropriate stake in the result so that they can evaluate what incentives are proper.
For example, you wouldn't want a corporation to enter into a contract with a strawman, who rolls over a lets a court declare that it is legal and binding, and then assigns the contract with legally invalid provisions in it to a third party who would not be permitted to litigate his rights because they were already decided in an advisory opinion.
It also keeps the courts out of the legislative process by having courts deal only with laws that have been passed and are ready to be imposed, rather than ruling on proposed laws that would inject the courts into the legislative process.
Incidentally, the law is the same in Australia flowing from Re Judiciary Act 1903-1920 & In re Navigation Act 1912-1920 [1921] HCA 20; (1921) 29 CLR 257 (16 May 1921): that is the High Court is not permitted to issue advisory opinions. For a recent discussion of this case see http://www.ruleoflaw.org.au/advisory-opinions-rule-law/.
Because the Federal Constitution gives the Commonwealth the power to confer Federal jurisdiction on State Courts (a power that has been executed) the High Court has held that the State Courts therefore form part of the Federal judicial system and they are likewise also prohibited from issuing advisory opinions. This is true even if the State Constitution says they can because Commonwealth law trumps State law. In theory, an advisory opinion could be sought for a law that only dealt with state matters but since the Commonwealth has wide ranging powers (including over corporations, weights & measures, currency, telecommunications etc.) it would be practically impossible to write such a law.
However, the High Court has held that the States (in the person of their Attorneys General) have standing to challenge the validity of a Commonwealth law where it impacts on states lawmaking ability even though there may not (as yet) be a case or controversy. This is because the lawmaking powers the Commonwealth are limited to those powers ceded to it by the States on Federation and detailed in the Constitution.
