While wanting you to buy a new keyboard may indeed be a reason for the battery warning it could also be a healthy dose of litigation fear. People are stupid. Really stupid. A “caution - use only rechargeable ML2032 battery” does not protect the company from someone sticking a CR2032 into the unit and causing a fire as much as “Do Not Replace Battery”. In the former the question “so, how do you protect the customer from putting in the wrong battery” can be a pain to answer and not worth the effort for a $60 keyboard. Putting on the Do Not Replace Battery sticker lets the company simply say, what part of Do Not Replace Battery did you not understand.
I agree that not making it easy to replace sucks but unless the battery is keyed to prevent misuse then… well… People are stupid.