The FBI seal, along with all other government seals, are public domain.
We paid for them with our taxes.
Yes, there are laws saying that you can't use the FBI seal to misrepresent yourself as an FBI agent, or acting on authority of the FBI, but you have a right to display the seal in other ways. Same as any other government seal.
Wikipedia is using the seal in an educational manner. Fully protected and legal use of that seal.