I was going to say that’s part of being “Pool Stored”, but
@Lake Placid beat me to it with a far more detailed explanation. I was going to go the simpler route of the higher they keep your TA, the more volatile your pH will be causing them to be able to try to sell you pH+ and pH-, and if they are really good at sales, Alkalinity down and up. lol!
Pool stores care about making money. Off you. They don’t care about your pool. If they don’t keep you coming back for more stuff, they lose money. If you stop coming all together, they lose a lot of money. That’s why they all offer free water tests. They get you in the door. I’d venture to say every member here has a Pool Stored story. Most of their own experience. If they don’t have their own, they’ll tell you thousands of others from the years here.
TFP cares nothing about your money. Sure, they like donations (aka supporters) to help keep the site running. But it’s not required. And TFTestKits IS NOT affiliated with TFP. They have a common name, but are two entirely seperate entities. We recommend his kits because they are the best you can get for the money and include everything you need to maintain your pool Trouble Free.
There’s one member here that literally stood outside the pool store and handed out cards to TFTestKits.net and mentioned TFP to people. Don’t know how long that lasted, or if it was even real… but sounds like a good story.

You’ll see, your TF100 will come with cards. lol.