Suppose that Alice has two kilos of Cocaine in her refrigerator, and then she breaks into Best Buy and steals a 72" TV. She gets caught on camera, but they're not sure it's her, so the police get a warrant to search her house. One officer goes into the attic and finds a 72" TV... bingo, she's gonna get convicted.
But another officer was in the kitchen, and found the Cocaine. No conviction. It's a legal search since they have a warrant, but the police can't use the Cocaine in the fridge as evidence, because they have a warrant to find a 72" TV. The fridge is too small for a 72" TV, so the police can't look in the fridge, and they can't use anything they find in the fridge as evidence.
Alice gets busted for stealing a 72" TV, but the conviction for Cocaine gets thrown out.
So, back to the wiretaps. Just because a wiretap is legal doesn't mean that all the evidence you get from a wiretap will be admissible as evidence.
Second, calling the wiretaps legal does not make them so. The courts might declare that they're illegal without a warrant.
Third, it's the executive branch that calls the wiretaps legal. We should be at least this specific. The executive branch can claim PRISM is legal, but only the judicial branch has the power to actually decide on the legality.