So, are you objecting because it's what the rules say, or is it a general objection to players never losing mental control nonmagically unless the player wants them to? If the former, cool. If the later, still cool - but why is it different than physical control?1. Players decide what their characters think and do.
2. Some spell descriptions say otherwise.
3. Specific overrides general.
All three things are in the rules.
Same answer. Let's say we're talking alcohol. If you want to impose penalties on dice rolls due to inebriation, go for it. The player will still get to declare actions, but if you require a dice roll to determine success then the alcohol may be a factor.
However, if you think the character should behave certain ways while drunk, in other words, that certain actions should be declared...sorry, that's for the player to decide. Get your own NPCs drunk and roleplay them however you like.