"It's showing the extreme (clearly unacceptable) end of the spectrum to ask someone how they decide where to draw the line."

That's actually very lazy. It's shifting the burden of "drawing the line" on the other side.

It's the "well, why don't we just kill toddlers as well? Where do you draw the line?" in an abortion argument. It's the "why don't we just give 12 year old's automatic assault weapons" or "why don't we disarm the police and military as well" in a gun control debate. It's the "So why don't we just tax corporations 100% of their income" in a corporate tax debate.