It would be an even happier state of affairs if the Master's-qualified teacher were free to take whatever salary could be negotiated between her and the school, without union interference.

No of course a Masters doesn't automatically equate to a higher quality, but she can't get a job at all because of it. Because of the Teacher's Union, she can't even waive that extra $2000. She would be grateful to take the base wage if it meant employment.

Having a masters degree "may" not improve student performance. Unless of course you are teaching math or science at the secondary level and have a degree in those subjects in which case it "does" improve student performance. Without having a close look at the data they used (which is now twenty years old) it's hard to say if they took into account teachers with masters degrees and classroom experience. As teachers are required to continue their education just to remain licensed it makes a lot of sense for teachers to get a masters degree even if it didn't improve their ability to teach. There is very little value in spending money on specialized instructional courses or seminars that count towards relicensing credits when those methodologies won't be used in the classroom in another three to five years or will be rebranded and resold by some other company that is the flavor of the day. A masters degree can also be a path to administration, counselor etc.

Try looking for a job as an “overqualified” applicant in private industry. If they are not paid more or treated better then it’s assumed they will quickly move on.