Equally, their planning against the bad guys might be more fun all around if they know the GM doesn't know what those plans are; that way they can engage in a fun battle of wits, and see who can outwit the other.
Shouldn't they be trying to outwit the bad guy NPCs? Not the DM.
If games come to a battle of quick wits at the game table, my Players will win everytime against me. To start with, I'll never claim to be a quick genius. But I also have how many NPCs and plots to keep in mind. And there's 4-6 Players and just 1 DM. If the game was a battle between the Players and DM, I'd just pull out the tarrasque and claim checkmate.
Wow, you're harsh. My players kick me out of the room all the time, and we have a very trusting relationship. They just like to see the look on my face when their secret plan catches me totally unawares. That's part of their fun, and I encourage it.
I'd prefer the Players discuss their plans in front of me. One of the things I enjoy about DMing is hearing the Players discuss the game.
But it also lets me make sure there are no misunderstandings about the situation..."No, there were only 3 statues in the room, not 4." "The pit was 20' wide and 10' deep, not 10' wide and 20' deep." "I never said anything about flying monkeys with the BBEG, Johnny said that as a joke."
It also lets me warn about how the rules work..."Your wizard would remember that
sleep takes a full round to cast." "They'll be a 20% miss chance fighting in the night darkness."
It lets me think of how the enemies will react to the "surprise" without actually being surprised myself and taking an extra minute to decide.
And yes, it sometimes lets me "double check" my BBEG's plans. If the BBEG is a 24-Intelligence 20th-level wizard with 10 years to plan, I don't think it is fair to him to be defeated because the Players/PCs realized using a common cantrip would unravel his entire scheme because the 12-Intelligence 10th-level DM with maybe 2 hours to plan didn't thoroughly consider every single spell in the book.*
[*NOTE: This does not mean that such loopholes could not exist in a BBEG's plans, but I'd rather that be the DM's choice to have it exist. Not because the DM's baby cut short his prep time with crying. Such a loophole could also exist by design as the "answer" to the BBEG's riddle-like scheme. But not because the DM wrote up the scheme at 11 pm after a tiring day with work and kids.]
Quasqueton