jQuery makes things almost too easy. As a result it gives confidence and power to a lot of people to write code that despite all sillines will still work. A professional JS developer would've debounced the scroll events and cached the selector results.
jQuery promotes bad practices. People assume $('something') is a cheap variable access, when in fact it could be pretty slowly executing several functions. So they don't bother caching anything.
I think by "promote bad practices" you might mean "abstracts complexity away from the developer".
This is true of just about any API. Nobody blames the API when their ORM layer generates a massive SQL query based on complex query input. Understanding the performance characteristics of the 'backend' is assumed.
$FOO in php is cheap.
$('FOO') in jquery is expensive.
I don't think using $ to signify "Run some load of jquery code" was a wise choice. Newcomers who don't know what is going on under the hood will regularly write loops with $('FOO') inside them, which is crappy code.
They're doing what they think is best, so I don't blame them. I blame jquery for making it look cheap.
Why on Earth you assume that someone starting to use jQuery will come from PHP background, and more over, why should that person think that $FOO in PHP is cheap?
I guess anyone thinking in terms of performance cost will be smart enough to understand what $() does in jQuery, especially when advice to cache selectors is on every wall.
