If you have a business that deals with spam texts or if you host a honeypot for whatever reason you are allowed to do it. You are allowed to receive spam and do whatever you want with it except sending spam over twilio itself because it's against their ToS and to protect their own core business from getting their numbers blacklisted.
if twilio can not differentiate between inbound messages that are being forwarded and outbound messages that are created by the twilio customer then their service can't guarantee to be reliable because someone could basically create a DOS attack by sending spam to any twilio customer expecting that twilio will blame their customer and suspend the account.
It's not their job to differentiate they are just a carrier they scan for outgoing spam to protect their own core business and to not get blacklisted everywhere. Read the ToS they are pretty specific about it.
but then they are simply not suitable for the use case of forwarding inbound messages because the risk is to great that i'll get shut down just because i am receiving spam.
They're receiving the spam because it's sent to the customer.
They're sending spam when the customer asks.
They're also deciding that that customer is a customer no longer because of the choices that customer made with regards to sending spam, not receiving spam.