Why would you take the ability to chose from the player specially when you put so much emphasis on player agency and customization? The only reason that comes to mind is to make things easier on people with low D&D knowledge and even so is a bad reason because you're picking your spells everywhere else in the game already so... I don't know, doesn't make any sense to me.
I'm sure Larian had their reasons. The point of Pact of the Tome is you get cantrips that Warlocks wouldn't normally have access to so maybe they couldn't get the game to offer access to those cantrips belonging to other classes only one time. I don't think they did this specifically for the intrinsic joy of refusing to give you choice. It can be difficult to always get things to work as intended in a videogame. But honestly, this is fine. That is a very good selection of cantrips.
If you take this to it's ultimate conclusion, how does any class get spells/abilities in the game?
Where ever that definition is spelled out, all they have to do is check mark or select Warlock and define at what level. There is no technical reason this isn't implemented. It is 100% a decision that Larian has made, for whatever illogical reason.
And this decision being so, so illogical, I would almost bet that it won't be like this in Release but it's obvious how this should work and such an easy fix.
