Who ever told you that is talking shit. External wastegates are the same as internal wastegates, only not part of the turbo. As long as it is plumbed back into the exhaust before the cat the exhaust gasses still pass through, therefore no extra emmisions are released.
A screamer pipe (vent to atmosphere) is illegal for 2 reasons. 1: Extra emmisions. 2: Noise. Sounds great though
They can yellow you for it, but I doubt they're fine you for it. I know plenty of people who have been stickered for having no cats, decat pipes, some have even gone over the pits like that. They fail, but it just means they dont get rid of the sitcker.