It's easy to fire "ineffective" teachers (quotations used to show the mutability and variability of the term, rather than sarcasm). Even with the supposed difficulties presented by tenure, so many school districts are struggling for money that it's a simple matter of invoking the RIF-hammer.

It's harder to identify what makes an teacher ineffective, and harder still to identify what makes an effective teacher.

Even harder still when lesson plans are mandated from up top and are seen as things to "deliver" (like a pizza, or a newspaper). I worked in a school district in which some of my lesson plans were written for me (down to the "Spend 5 minutes on this, 8 minutes on that" level), and was evaluated (on those days) based on my ability to comply with them to the letter. One of the comments I received on an evaluation was "We were hoping you'd show more creativity in your teaching" of a lesson that was assessed on how well I followed a script and how well I got students to follow the script.

Frankly, I think the thing that will truly reform education is an entire change in mindset away from students "doing well on tests" and toward students doing good and interesting and challenging work.

That said, your notion of attending professional development is good not only for struggling teachers, but for everyone. It's stupid to expect teachers to improve if the only thing we give them is a carrot or a stick.