It wouldn't be particularly hard for a trained person to block their thoughts to a brain implant, if the police were trying to read their mind.
I wonder what happens to these BCI results when the user is on psychedelics a la MK Ultra
EDIT: Wait your point was slightly different then I first read but that makes perfect sense!
interrogator> We have been torturing you for 10 hours. If you spell "John" we will stop torturing you and torture John instead.
interrogated> [The pain is too much. We have a deal. Sorry John.] JOHN
A better interrogator can be more subtle, like
interrogator> We have some evidence [1] that John is the one that put the bomb, and that you are innocent, but my boss is not sure. If you confirm this info we will send you immediately to your cell.
[1] It's a lie! Also, John is innocent in case you are wondering.
Currently it requires an invasive and risky brain surgery which AFAIK would be a definite no-go. At least in the US, this would likely be considered extremely inhumane unless it could be done without such significant risk of death or brain damage - given the nature of the surgery I find that unlikely.
In the cases where an invasive brain surgery would be permitted, I imagine torture would already be an option. Keep in mind torture doesn't have nearly the same risk of death and permanent brain damage as an invasive brain surgery.
The difference is on the level of hearing someone's chess moves dictated vs. reading their mind to understand how they perceive the game.
It is entirely a consciously composed and intentionally sent message.
You're assuming that everyone's motor cortex/nervous system is wired exactly the same way. Given what we know about the variability of the human body, I wouldn't expect this to hold for the entire population.
Every human is wired differently. You need to train an ML model to read signals specific to that human for this tech to work.