Lately I've heard people using what I think to be a negative response to indicate a positive affirmation, like so:
You can touch the basketball rim? Well so can't I!
Person A: "Dave can't go today, he's feeling sick."
Person B: "Yeah, so isn't Mark."
In example one, the speaker is re-affirming that he can touch the rim. In example two, Person B is confirming that Mark is also sick. So why can't and isn't instead of can and is?
If this usage is incorrect, is there any root to its prevalence?
For background, I live in New England.