But they deserve it when the manufacturer has one of those enterprisey sites where you need to go through 10 searches to maybe reach your manual, when the 3rd party site just shows it directly.

Not really, and the third-party sites almost never show the PDF directly without first trying to harvest your email or phone number or subscribe you to spam, sometimes they try to steer you towards unaffiliated 800 numbers tricking you that those are associated with the manufacturer, sometimes they bundle the download of manufacturer's PDF with malware, browser cleaner app installers etc.

Sometimes the third-party sites are helpful and benign, sometimes they are merely spammers trying to upsell you, occasionally they are malicious.

Agreed, the manufacturer site behavior is also annoying.