"is bad about" idiom/phrase can be either a negative or positive assertion?

I was chatting with a friend who is a proficient but non-native speaker of English, and a misunderstanding came up about my usage of a phrase of the form "<something> is bad about <doing something bad>", with the intended meaning that the thing in question commonly does the bad thing. e.g. "Willow trees are bad about causing damage to water lines".

My friend's point was that doesn't make sense, because if they are bad about doing that, that logically means they don't do it. So I got to looking/searching/thinking, and it seems like there are examples of either case, where that form of phrase could mean that a thing commonly happens, or a thing commonly doesn't happen.

For example, "They are bad about brushing their teeth" vs. "They are bad about not brushing their teeth". In my mind, both phrases mean the same thing - that they don't commonly brush their teeth.

Does the meaning of the phrase depend on whether the action/effect is perceived to be good or bad? So either a thing perceived as good doesn't commonly happen, or a thing perceived as bad commonly does happen?

Or I guess maybe another way to look at it is that the phrase is always used in the sense that has the negative connotation?

A few other examples of similar phrases, for reference.

Women, too, are bad about prying into the affairs of men.

Conversation with the High Priest of Coosa, Charles M. Hudson, page 7, books.google.com

Human eyes are bad about misjudging tall objects.

Shotgunning: The Art and the Science, Bob Brister, page 136, books.google.com

I don't eat the pears off that tree because they are bad about having body hair in them.

Stories by Charlie: Mtn. View Arkansas, Charles E. Forte, page 62, books.google.com

Solution 1:

For me, a native BrE speaker, Tommy is bad about brushing his teeth. communicates information about Tommy; the frequency of his brushing is inferred. It implies that Tommy (most likely) lacks the discipline to brush when he should.

Hence, suggesting that a willow tree is bad about doing this or that intimates that the willow tree could do otherwise, which it can't.