**IMO** Not trying to be rude in any way. xD
Large scale breeders who supply fish shops with their bettas purposely cross breed different lines until it is a spider-web like jumble of genes in each and every betta. There will be plenty of "surprises," unpredictable colors in the fry, and random tail types (which has taken breeders decades and decades to create) and most importantly- deformed fry. Because of the sporadic and mass scale breeding, deformed and weak fry grow into deformed and weak adults. (Not all, of course, but a large number within a single spawn.) This brings me to the culling, I find it interesting how you say
"By keeping every single fry-you are not doing yourself nor the line you are working with any favors......
One of your goals should be quality not quantity"
That's the stickler- quality, not quantity.
Low quality x low quality = low quality.
High quality x high quality = high quality.
The number of fry that need
to be culled from a good pair is far less than that of a pet store pair. Most novice breeders don't even want to cull their first fry, let alone even know what to look for when culling. This leads to deformed fry living a miserable life, and again, weak adults.
I agree 100% that it is a great experience, and that people should be responsible with it. That's why good quality stock is probably the most important step you'll take in preparation of this hobby. It takes as much energy, space, time, water, supplies, money and patience to breed low quality as it does high quality. You can also make a profit on higher quality- not so with pet store offspring.