Whats wrong with a company who would rather have their heater installed by a competent installer? I'm sure that YOU can install your heater. But your ability is not the same as all the other DYI'ers out there. Iv'e seen my share of scary installs out there...
If you were selling a product that has a "moderate to significant" level of liability associated with just the install alone, Would you want to pay out warranty for things that wouldn't even be an issue if you, or someone you trained, installed it? How about warranty-ing the same thing over and over for a year, or however long you dared to make your warranty, because the install was incorrect, causing the part to continually fail? All the while the customer/ DIY'er telling assuring you that the install was "good to go"?
The manufacturer is not trying to punish you for wanting to get the best deal possible, they are covering themselves from bleeding (as explained above) to death, or even worse, sued. BTW, that line has always been in the warranty, even before the internet. It probably wouldn't be an issue if not for the fact that you can not only kill yourself, but your whole family if something goes wrong with that heater due to an incorrect install.
I have no problem with you buying a heater on line. Its your right to shop the best deal. I will even do the competent install for you. Do i have a different scale for internet installs? Yes, and why shouldn't i? You are still getting a heck of a deal than what you would have, had you supported your local mom & pop or more PC, "brick & mortar" shop. Those online guys sell those heaters to you for less than i can get them!
"That's all i got to say about that..."
Forest Gump
To be honest, i don't see many Hayward heaters, so i don't know of the longevity of them. I would have to think if EVERY heater they made had problems, they wouldn't be around for very long. And I have known of Hayward throughout my entire pool industry career. So i suppose they can't have a horrible product.