It's being done so they don't "oversell" and so a reservation is issued with every ticket.
It's not genuinely compulsory, you can buy a walk-up ticket "for another train" and use it on the one that is "full".
It's actually fairly sensible to be able to restrict sales of "trains" in this sort of way, particularly where timetables are varying a lot so you can open a train when you know you'll run it. Long term it could do with being a different flag so it isn't misleading. Perhaps the "reservations recommended" flag should be adjusted for this behaviour, or a separate "sale only with reservation" one.