You're right, the text doesn't mention the scenario of a two-handed weapon being used.
However, given the actual text, and the specific exclusion of crossbows and bows from penalty, and the lack of any mention of other two-handed weapons, I would suggest that one of the following is true.
1. Two handed weapons (excepting those specifically mentioned) incur a -1 penalty (such weapons are assumed to be used in the off-hand as well as the on-hand)
2. Two handed weapons (excepting those specifically mentioned) incur no penalty, but still stop the buckler from providing an AC bonus.
3. You may not use two handed weapons in conjunction with a buckler.
Personally I'm in favour of either 1 or 3. 1 because it makes sense (the additional weight of the buckler would inhibit any use of the hand it is on) and 3 from a pure rules viewpoint (the buckler gives specific exemptions for weapons in the offhand, and bows and crossbows, but not for anything else, and thus follows the standard rules for shields otherwise)