I think the point is that when teachers start having well paying secondary jobs, the actual teaching may suffer. If you have an hour after school, do you spend it helping your students, or tweaking your latest for-profit lesson plans?
If I'm well paid and satisfied with my current job then I'll make sure to get a good night sleep and be ready the next day to do a good job. If I'm unhappy or think I could be making more then I'll either do work on the side or spend more time working on my resume and interviewing for a better job. If I think my career doesn't have much potential then I'll really spend a lot of time outside of work trying to switch careers.
If you want teachers to do a good job, hire talented people and pay them accordingly. If you want a McDonald's like education system where a handful of chefs come up with bland but easily executable recipes then we can go down that path too. Just don't be surprised when your kids are switching careers from fry-vat guy to third grade teacher. And don't be surprised when the talented teachers see it as a stepping stone and get the heck out of there.
Until schools are palaces, and teachers earn high six figures, I have no problem with 1) teachers earning extra income, by 2) sharing knowledge with other teachers.
If you want to make the world a better place, then argue that all teachers should have a budget to buy lesson plans from places like this.
...so that they can focus their time and energy on the students who need help, rather than on creating lesson plans, over and over again, all across the country, for the same topics.
Remember that even with what they pay now, teaching positions are in high demand. Quadruple what teachers earn, and you'll have even more demand. How are you to be sure you're getting what you're paying for? In medicine and law, that's achieved through a series of very expensive gate keepers.
In any event, I think it would be great if a young hotshot from Harvard could be paid $90,000 to start at some godawful but trying to improve school, with a 50% bonus potential if his students show improvement. But the unions would never allow such a program.
It goes without saying seasoned teachers will have a cache of lesson plans that they can, and do, pass along or sell. Why should one not be allowed to profit on the labor they already expended for an incredible service to other teachers?
This has already been "adopted widely", for decades, and the teacher-lesson-plan-community is an incredibly active and vibrant one. The countless lesson plan networks, online/offline, free/paid, are an invaluable service to teachers.
I find the negativity odd, that we as hackers, who create, build on, share, and sell our toolsets, would have a problem with this.
For the most part, you're getting additional income for little extra work, and what work you do to improve your side job comes back to improve your day job as well.
If you're spending hours struggling with a system that is difficult to set up / get it to pay out / otherwise distract from teaching, then sure. But I haven't used the system so I couldn't say whether or not issues like this are occurring (and it certainly sounds like it's working for the teacher in question).