Also, it's worth noting that this is usually done with good intentions: News outlets are highly incentivized to be the first one to break any particular story, so if a press release is not embargo'd, everyone will rush as fast as possible (on the order of minutes) to make it into a news alert. Even assuming the best possible reporter, such an article is bound to be poorly researched and edited because it is being rushed out. By putting a press release under a few hours of embargo, news outlets get a bit more time to prepare their coverage before it goes live.

(Please don't reply with specific examples where this standard is not upheld. I'm clearly not arguing that this strategy is 100% effective.)