5 years ago: "but it's profitable enough to hopefully hire another guy or two this summer."[0]
The founder has completely bootstrapped the site since day 1, and has done an AMA on Reddit talking about the whole situation[1]. Ads are absolutely necessary to make it profitable, but clearly taking investment now means they are going to squeeze every ounce of dollar out of the site and hence the sentiment of "aggressive monetization".
[0] - https://www.reddit.com/r/IAmA/comments/eicjf/im_the_imgur_gu...
Point being: unless you have been directly involved in the decisions accusing them of either malevolence or greed is a pretty tough accusation to make. Sometimes (usually) it's cold hard business realities that drive people to adopting these sorts of monetization strategies.
So now the cycle will repeat. Someone will start a host that simply serves images. People will use it. They'll add some ads, people won't care much. They'll start serving more aggressive ads and someone will start another host... over and over again.
How? I don't see a way from Imgur to monetize on sharing images directly.
If I can see you viewed an image referred from /r/woodwork I might offer you some woodworking tools and drive up the conversation rate of my advertisements.
It is. The wealth of images alone, the sheer amount, from memes to vacation pics is worth so much now that image recognition is taking off.
They did it by building a community around the non-reddit related traffic. Basically redditor's would share imgur links outside the reddit ecosystem (Facebook etc), then other people would visit imgur directly instead of going through reddit. These secondary users are monetizable.
Imgur had to eat the costs of supporting reddit's hotlinking (though not all reddit posts are hotlinked) but that was more or less imgur's "marketing budget."