The teaching exception means that a teacher in a classroom can show a copyrighted piece as part of her lesson plan. It does not mean that you can "package" a lesson plan that includes the same materials, then sell them to other teachers.
If it were possible to use the teaching exception in this way, all educational book publishers could use any image they want without paying for it. I've worked with a children's educational book publisher for the last 10 years. They have to buy all copyrighted materials used in their books.
A caveat. I've never seen a TPT lesson plan, so maybe I'm unclear on how they "package". Specifically, I use the term package to mean that the actual copyrighted materials are included with the file you receive. TPT could get around the copyright issue by inserting reference placeholders, then instruct the purchaser to acquire the images on their own, but that's a significant amount of work. I would be surprised if that's how they were packaged.
The linked article points out that Scholastic bought the site from it's creator and then sold it back when it didn't become an overnight viral success.
"Kindergarten Teacher Earns $700,000 by Selling Lesson Plans Online"
May 2012
http://www.macon.com/2012/06/12/2056692/north-carolina-compa...
It seems a natural progression from generating Bingo Cards, to branching out to lesson plans to facilitating the sale of user-generated lesson plans.
Standardizing the lesson plan eliminates (or greatly reduces) the ability for the teacher to be creative in their classroom and teaching. On top of that, a standardized lesson plan would also be hard to work around in case there were areas that took the students longer than expected.