Finally, when it comes to character freedom of action, no discussion on the use of the mind would be complete without something being said on mind control, which has a long and traditional history of use in both RPGs and fiction.
To start with, in D&D, we have items, monsters and spells that all enable the DM to control the player's actions. A party will never hesitate to use spells or potions and the like against monsters, and we all love to turn the monsters against themselves. In kind, a party encountering a magic user, a nixie or siren, among a wide host of other monsters, expects to suffer a bit from the charm spell. It isn't always pleasant, but there are plenty of opportunties for a DM to step in and tell the player characters what they have to do, because they been charmed, or suggested, or magic jarred, or what have you.
It isn't as though the whole mind control technique hasn't been employed for centuries to keep things interesting. Merlin being enchanted by Vivien. The fun and games of A Midsummer Night's Dream. Mina Harker from Dracula. The Emperor Ming from Flash Gordon. Heinlein's The Puppet Masters. McCoy and Sulu in Return of the Archons. The Penguin. The Imperius curse.
Over and over again, characters get overwhelmed by the dark, unforgiveable influence of the evil forces at work in the world. It isn't fun, but now and then you don't get to say what happens to your character ... and them's the breaks.
A DM, obviously, has to be very careful how this kind of scenario is played out. He or she can't simply rope a party by having a magician show up and turn everyone into mindless zombies. No, first a tale has to be told, which includes words like 'danger' and 'curse,' and preferably has some reference to the tendency of the evil entity/device to control the minds of people in contact with it.
Then, preferably, the party should have to willingly travel overland towards where the entity/device is, thus willingly making the choice to either ignore, or intelligently challenge the entity/device in its lair.
If, however, the lair is approached, and if the party has gotten there of their own free will, then the DM is fair to judge that Pandora's Box has indeed been opened. It's no use trying to stuff all the bad back into the box. It is too late. The DM is at that point perfectly justified in turning the party into zombies.
My personal feeling is, however, that the DM is also responsible for offering the party a way out. There has to be an end game that neutralizes the effect ... and fairly quickly, since otherwise the campaign is going to get awful frustrating and boring.
Mind control in a campaign is a difficult adventure to run. It involves trust - mostly, the players trusting that the DM isn't going to just fuck around with them for fun. Given the general behavior of a great many DMs, this is a very difficult trust to earn. Parties are notably gun shy. And that's why I say, be sure the party recognizes their part in making the mind control happen, and get them out as quick as you can.
And while the adventure is happening, tell your party to sit back and enjoy the ride as much as they can, with their hands inside the car, until the 'coaster comes to a complete stop.