That's the case every season regards Season Tickets and the cash generated from that source is used in all sorts of ways not directly related to matchdays. Its the additional income from cash paying fans on a match by match basis that will be used to cover/partly cover the actual cost of opening the stadium week to week.
Last season our average home gate was just under 6000 , including visiting fans. Allowing for ST Holders that leaves around 2000 punters paying cash. At £15 a head (and I think I am underestimating) that works out at a minimum of £30000 per game cash income to cover costs. £570, 000 a season. At £20 a head that is over £750000 a season.
Motherwell have likely lost that cash anyway this season, but to add to that loss by opening the ground long term for no financial return is madness. And I am pretty sure you would not be happy if attending games was limited to cash paying customers only in an effort to restrict losses.
And as for losing income from Season Ticket holders by giving fans the option of carrying unattended games forward to next season, it is exactly that...an option. Having watched the games online I am positive many if not most fans will not take up that option. In my opinion Motherwell have handled the situation extremely well and that includes looking after Season Ticket holders.