I came across an explanation from a Southwest employee about why they are strict about people not moving seats anymore.
The short version is their system now assigns seats and calculates aircraft weight and balance ahead of time using multiple cabin zones. Because of that, if people randomly move forward or backward it can throw off the planned center of gravity.
Another part of it is the extra legroom rows. Those cost extra now, so they also do not want people just sliding into them after boarding.
I get the safety explanation, but it still feels a little strange for an airline that used to be known for open seating. Personally I still do not think any seat on Southwest is really comparable to business class like the employee mentioned.
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