No, you're misunderstanding what's happening. If you unregister your phone number from iMessage, then that's the end of it. People sending messages to your phone number will get SMS messages. It is impossible for a de-registered phone number to receive iMessages on an iPad. What's actually happening is that the people who are sending you messages are sending them to your Apple ID instead and iMessage is always registered to an Apple ID so the messages are delivered to your iPad. It's not swallowing anyone's messages from people's iPhones because that's literally not possible.

It's not a dark pattern. It's not an illegal anti-competitive strategy. It's a successful delivery of a message to the recipient address it was sent to that you're simply misunderstanding.