Online tickets less than 90 minutes before departure

[danm14]

Is there any sensible reason why tickets cannot be purchased online less than 90 minutes before departure? It appears to be a means to impose a (possibly very significant) financial penalty on those who need to travel at short notice on the last train of the day?

If I need to travel with less than 90 minutes notice on a train other than the final train of the day, I can simply purchase a flexible ticket for a later train and use it (minus seat reservation) on the train it's too late to buy a ticket for.

But if I need to travel at such short notice on the last train of the day, I can't do this - the first train I can book is tomorrow so the ticket wouldn't be valid today. I must pay the full walk-up fare, which could be more than double the price of the most expensive flexible online ticket.

I can purchase a ticket online for the very same train from a station further up the line, as long as the train leaves that station in more than 90 minutes. But it's a breach of the conditions of carriage to use more than one ticket on a single train if it saves you money, so doing this and just paying the walk-up fare to that point isn't allowed.

I seem to even be able to purchase a ticket from a station that same train is due to leave in more than 90 minutes, while the train is already moving which makes a mockery of the excuse I'm sure I've read somewhere that closing bookings a certain time before departure from origin is to ensure seat reservations are displayed.