Yes that this should be stamped out immediately. Club should have measures in place to prevent this. This new 70% thing does nothing whatsoever to stop this, as it's simply how many times the ticket is used or forwarded, and not down to people illegally entering the SOL.
It already does and has just announced further SC checks at the turnstiles.*
As for the 70% issue, you've totally grabbed the wrong end of the stick ...
... it has absolutely nothing to do with entering the SOL.
* 13th Feb 2025
Sunderland AFC supporters are advised that ticket checks have been introduced throughout the Stadium of Light.
Fans are reminded that all match tickets and season tickets (card or digital) must correspond with their correct age and status.
In recent weeks, spot checks have unfortunately resulted in several fans being escorted from the stadium after entering the ground with tickets that did not align with their correct status.
These instances have taken place in general admission and premium areas and included adults entering with concession tickets and personal assistants entering without their dependent.
Club policy states that any person(s) identified entering the stadium using an incorrect ticket will be escorted from the ground and provided with an opportunity to purchase the correct ticket, including match and season tickets.