It seems totally reasonable if you're not operating a company to be altruistic.
* Pushing all the rendering to the client makes development easier, eases the transition to native apps, and uses fewer resources on the back end.
* The fancy site drives more conversions and makes the stakeholders happy.
* Not having fast internet is a crude filter for disposable income and losing those users probably goes unnoticed and might even increase the value of ad placements.
(I'm not putting /s because there's actually people that think this is a reasonable opinion in the general case).