I think it happens because tactics is based on total skill level - so with 0 levels in the skill and a negative modifier (for tactics the relevant modifier is actually learning IIRC) you can have a negative total skill level. You can see something similar with two-weapon fighting if your dexterity is low enough. The penalties make some amount of sense, especially in the case of two-weapon fighting. On the other hand, other skills based on total level like find weakness don't have penalties for negative total level, so it's hard to say if this behavior is intended.

I have a fighter character now with a tactics skill, and he has bonuses at 'normal', so I guess it's intentional after all. Seems a bit counter-intuitive though, to list those bonuses under the skill setting? As if the skill gives bonuses even if you don't use it, I dunno.