They will make it illegal to manufacture, detain or sell parts made to defeat the emissions. That's why they will start by attacking businesses, not end users.
When you cannot make and sell a catless/DPF-less exhaust because it's illegal, you'll have way less people with deleted vehicles. And if you have less people buying them, you'll have less revenue, and it'll lower the attract of the market. I guess.
The RPM act is here to protect the people who want to modify vehicles to go racing, which currently aren't exempt under the Clean Air act. Basically, even if you want to go racing on private grounds, you're not allowed to modify the emission systems under the current regulations.
You have to remember that not everyone is able to fabricate their own parts, and the Clean Air act also concerns the ECU tuning. So even if you were to physically remove the emission devices, tempering with the tune to make the engine run without them is also considered illegal.
Same as Europe basically.
PS : If you look at most emission delete and tuning platforms, they always say "for offroad use only". EPA said this wasn't deterring enough people currently so when they passed the Clean Air act they used a broader spectrum.
PS2 : from what I understand it also maters in case you purpose build a vehicle (think tube buggy) because you still aren't allowed to remove the emission devices from an engine you'd get from an EPA regualted vehicle, IE using an LS3 from a Camaro without the Camaro exhaust etc. If you were to build you own engine from scratch I guess you technically aren't concerned by the clean air act, but I'm not sure how many people are in this case.