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.