"Sirius might have volunteered to take the potion had he been given the chance, but he was never offered it. Mr. Crouch senior, power mad and increasingly unjust in the way he was treating suspects, threw him into Azkaban on the (admittedly rather convincing) testimony of many eyewitnesses. The sad fact is that even if Sirius had told the truth under the influence of the Potion, Mr. Crouch could still have insisted that he was using trickery to render himself immune to it."