If you write a lesson plan as part of your work, that is for your job, unless there is some other agreement with your employer, it is the property of the employer. It is work for hire. Period. It doesn't matter if you do it on your own time, on your own computer, at home, unprompted, on your own initiative, any more than a memo that a sales manager writes at home belongs to him or her, or an account manager that goes above and beyond on a sales PowerPoint owns it.
If you think teachers should be exempt from this you are arguing that teachers should have privileges that other professionals do not. The fact that teaching is generally a tough, underpaid, unrecognized profession does not enter into that basic fact.
The hazier question is actually "Are lesson plans copyrightable?" In case you forgot, I Am Not A Lawyer, and this one is tough to Google, since you tend to get lesson plans about copyright instead of information about copyrighting lesson plans. My guess is, however, that lesson plans should play out like recipes, copyright-wise:
Mere listings of ingredients as in recipes, formulas, compounds, or prescriptions are not subject to copyright protection. However, when a recipe or formula is accompanied by substantial literary expression in the form of an explanation or directions, or when there is a combination of recipes, as in a cookbook, there may be a basis for copyright protection.
Protection under the copyright law (title 17 of the U.S. Code, section 102) extends only to “original works of authorship” that are fixed in a tangible form (a copy). “Original” means merely that the author produced the work by his own intellectual effort, as distinguished from copying an existing work. Copyright protection may extend to a description, explanation, or illustration, assuming that the requirements of the copyright law are met...
Copyright protects only the particular manner of an author’s expression in literary, artistic, or musical form. Copyright protection does not extend to names, titles, short phrases, ideas, systems, or methods.
Along these lines, regarding lesson plans written for work, the teacher does not control the copyright of that particular expression of the ideas, ingredients, systems, etc. There is, however, no reason the original author, or anyone else, can't create another version of the same lesson and sell it online. For that matter, there is nothing stopping someone else from making another implementation of the lesson they buy online and doing whatever they want with it.
This makes common sense, fits with the way things actually work in the real world now, allows teachers or schools to pursue a moderate but not monopoly rent on the content, and seems perfectly consistent with a plain reading of the law.