If a billboard company accepted an ad that included a threat on the president’s life or recruitment info for a known terror organization, are they complicit in the crime? Water is a basic utility so I don’t think that’s a fair comparison

This is more like a firearms dealer selling a gun to someone after they put their intended usage as “robbing banks” in the ATF form

how does anyone not know where the line is?

An example that makes it more clear: "by that logic it's my fault that i was robbed for leaving the door to my house unlocked."

No, it's the robber's fault you were robbed. The robbery is the illegal part. It is not illegal to leave a door unlocked. Back to your train wreck of an example: it is not illegal to sell keyboards, and it is not illegal to provide water to people. Extortion is illegal. Denial of Service attacks are illegal.

That's where the line is. It is the border between legal and illegal.

Obviously we need to go after supermarkets and corner stores since criminals eat, so somewhere past that.