And the REAL reason why they do it, more marketing upon the common man who literally knows nothing and how these rumors get started. The much bigger profits.
You do NOT want to disturb wear particles that have gotten stuck in running sediments, that is what stirred up then fails parts later. What on earth does the magnet do there??? It doesn't take enough thinking to heat your hair oil to be able to translate that idea to engine instead of trans and insert 'seafoam in oil' in place of the word flush. Or the question I continually ask, just WHAT IS IT you are trying to clean out of the engine?????? I say leave the trash stuck where it doesn't hurt parts. Flushing is a response to not doing work that should have been done long ago.
The PCM really cannot compensate for anything inside trans wearing except line pressure possibly, it cannot make up for any true wear on parts, the initial build clearances are set to allow for the most wear possible unless you have a rebuilt trans, the trans shops generally slap all that back together without really resetting clearances for max life there. It costs them way too much money spending that kind of time on them. Why I do all my own ATX and haven't had one last less than 10 years after rebuilding it. For instance the 2-4 band on these uses a special length band set screw that comes in different lengths, you measure using special tools and change the screw based on the clearance of band. You can buy like a $4 dollar bolt and nut and then make your own adjustable band like in the old days like I did, these are not adjustable except for changing the screw, somewhat more difficult. The clutch packs get set to a nominal clearance and the servo piston the same. Nothing the PCM can do to make up for any of that being off.
If one thinks the wife driving the car has changed the trans I suggest examining the thinking process there. I switch with others on three different PCM trans cars a month or more at a time and no difference there at all.
Trans fails right after the new fluid change because it was minutes away from failing anyway. Why it began to act up. Human nature being what it is, the owner then blames the fluid change being reluctant of course to think that his forestalling of changing the fluid to begin with is what caused the problem. It cannot be my fault, therefor it must be the fluid, or the big fakeout to self that causes so many other problems. Fix the driver first.................and 2/3 of the car problems disappear.