Season ticket holders need to be given sufficient time to claim their seats hence why they have until end of Monday. After that, there simply isn't enough time to stagger the ticket sales based on member points.
If they were to put tickets on general sale only at the last minute then this will significantly reduce the attendance and also create problems for the ticket office in the last minute rush.
If you read the club annoucement from the Reading OS it states that "Selling details for member card holders will be confirmed later today"
I have an idea this will be on a points basis after Monday.