Earlier quoted context omitted.
Why? It's effectively the same action as placing a new order. Cancellations are, for instance, how lots of market makers alter their prices in reaction to events.
If I understand correctly, these are planned orders that are un-planned days if not hours before they go out to the exchange, not orders that are sent to the exchange but cancelled before they can be filled. The latter is what the market makers are doing. So it is more effectively the same as deciding not to do what you have been doing every month. It would be impossible to enforce a prohibition against that, as that…
So? That hardly seems impossible to enforce. You just don't allow cancellations unless they are also planned. If you're allowed to schedule recurring sells, say, 30 days in advance (I don't know what the specific rules are), you should only be able to cancel them with 30 days advance notice.
The whole point of the planned trade exemption is to allow trades to happen independent of insider knowledge. If you allow cancellations, you're allowing insider knowledge to impact the trading.