Um - well, I give away my lessons for free, as I work in a public school. The government pays me - it's a social good/social contract kind of thing. Taxes pay my wages - just like they pay the wages of the police, the nurses, doctors, garbage collectors. I assume that my students are going to take my ideas, add them to their own experiences, combine them with the teachers, coaches and family that they have around them as they grow up and then create, design, build, write their own ideas.
NO teacher works in a vacuum - all teachers will use, reuse, reshape ideas.
I use the internet and content on the net to inform and put together the majority of my lessons. I expect my students to do the same - there's a case that could be made for leaving them all at home and telling them to use google to learn. There are dozens of websites where you can download worksheets, pdfs, lesson plans - many set up by teachers. Most homeschooling parents will do the same.
Most teachers don't expect to make money off their lesson plans, but they are their intellectual property - and so if I started putting my materials up online, I'd slap a Creative Commons notice on them - just as I do on my flickr page.
If you're talking about making knowledge propietary and enforcing copyright of ideas - that's another topic. There is grounds for it I guess - but if you look up
iTunes U - or
MIT's Open courseware or even
TED - knowledge is getting more open, more available and oddly enough, the majority of it is FREE.