Agreed, it would be much better to have one of those lists like at airport check-in. Does your luggage contain any guns, explosives, etc? Rather than rely on the staff at the local Spar to interpret the rules.
The argument is that loose batteries are more likely to be packaged incorrectly and so short. I'm thinking particularly of unprotected lithium cells with tabs or wires. Lithium batteries integrated into equipment typically have a BMS that cuts them off if they get too hot or drain too fast, which loose cells don't. Also LiPos as used for model aircraft etc can be quite a bit more volatile than cells used in phones.
Theo