Using "so isn't" or "so can't" instead of "so is" or "so can"?

Lately I've heard people using what I think to be a negative response to indicate a positive affirmation, like so:

Example 1

You can touch the basketball rim? Well so can't I!

Example 2

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.

Solution 1:

This is widespread, but relatively rare; it's called the so don't I construction.
As it happens, I was one of the first to study this construction, since it's part of my idiolect.

Briefly,

• the negative is essentially spurious, and has no meaning
• the construction is restricted to so-clauses with Subject-Auxiliary Inversion
• the construction is a tag, and echoes an auxiliary from a previous clause, like a tag question
  He's leaving tomorrow, isn't he? ~ He's leaving tomorrow, and so isn't she.
  (this is also the way normal non-negative so clauses like and so is she work.)

Further details in the links.

Executive Summary:
Don't worry about it. English is more various than you expected, that's all.

Solution 2:

John Lawler nailed it.

The only thing I would add is that this construction usually connotes sarcasm. I'm also a New Englander, and also grew up using it. I still said "so do I" and "so can I" all the time; when I said "so can't I!" it implied that your statement was somehow ridiculous, usually because you're stating the obvious or bragging about a humdrum achievement.

"I can bench press 50 pounds!" "Yeah? So can't my grandmother!"

In fact the most common occurrence of this construction, in my experience, is in statements like "so can't anyone!" or "so isn't everyone!"

If you follow the link Mr. Lawler gave, the idea of "implicature canceling" meshes nicely. Saying you can do X implies that other people can't. The negative in "So can't I!" cancels that unspoken implication. Usually (in my experience anyway), that situation calls for sarcasm. A linguistics professor pointed out that the construction is used much more by children than by adults, and I offered the speculation that that's why: it's a bit impolite because of the implied sarcasm.