I'll begin with the concept of Signing Statements.
We only have one law-making branch: Congress.
The president may propose laws. The president may negotiate with Congress over the passage of laws that he deems necessary and proper for our country. But, only Congress can write laws and pass the laws.
When Congress has passed a bill, it presents the bill to the president. The president has three choices: sign it, veto it, or do nothing.
If the president signs the bill, it becomes law.
If the president does nothing, it becomes law after 10 days unless Congress has recessed making it impossible for the president to return the bill to Congress within that 10 day period.
If the president vetos the bill, he must return it to the originating house along with his objections for the record. Congress will then consider the president's objections. However, Congress has the power to override the president's objections.
At one time, Congress gave the president the power to sign the law and thereafter to issue a line-item of veto of any parts of the law that the president objected to. The line-item veto was declared unconstitutional. The president may not pick and choose what portions of the law that he will approve or disapprove. The Constitution only gives the president the option to approve the bill in its entirety--or reject the bill in its entirety. The president has no constitutional authority to rewrite legislation through the use of a line-item veto. The whole power of writing laws belongs to Congress alone.
The president's constitutional duty is to faithfully execute the laws of the United States--as written by Congress. The president had the opportunity during consideration of the law, BEFORE it was made law, to negotiate with Congress. If the president lost the political battle during negotiations, he cannot effectuate what he wanted from his failed negotiations through a post-hoc signing statement. He is violating his duty to faithfully execute the laws.
When a court applies a law, a court uses the plain language of the law itself. When the law clearly states that the government shall not engage in the electronic surveillance of United States persons without FISA court approval--the law says what it means and it means what it says.
If the law is ambiguous--capable of having more than one reasonable interpretation--courts will often look at the legislative history to determine what interpretation of the law best effectuates legislative intent. A presidential signing statement cannot alter or amend the law in a manner inconsistent with legislative intent. Any alterations or amendments to the law must be made by Congress alone.