I think you are asked upon installation whether you want to allow whitelisted ads (this was how it worked the last time I used Adblock, anyway; I use uBlock now). Some users want this because they see viewing ads as a way to support the "free" websites they use.
You can find more information here: https://adblockplus.org/acceptable-ads
Copying from that page, the rules for ads to not be blocked include:
1. Static advertisements only (no animations, sounds or similar)
2. Preferably text only, no attention-grabbing images
Ad placement:
a. Ads should never obscure page content (e.g. require users to click a button to close the ad before viewing the page).
b. For pages featuring a reading text ads should not be placed in the middle, where they interrupt the reading flow. However, they can be placed above the text content, below it or on the sides. The same applies to search results pages: paid search results cannot be mixed with organic results.
c. When ads are placed above the content of a main page, they should not require the user to scroll down. The available vertical space is likely to be at least 700 pixels. Advertising should not occupy more than one-third of that height. Paid search results on search pages are allowed to occupy more space, but they should never outnumber organic results.
d. When placed on the side ads should leave enough space for the main content. The available horizontal space can be expected to be at least 1000 pixels, and advertising should not occupy more than a third of that width.
3. Advertising should be clearly marked as such with the word "advertising" or its equivalent, and it should be distinguishable from page content, for instance via a border and/or different a background color.
4. Marking and placement requirements do not apply for hyperlinks with affiliate referrer IDs embedded in the content of the page. Additional criteria for hyperlinks with affiliate referrer IDs:
a. Redirects originating from the hyperlink should not present any other webpage than the destination page.
b. In texts, not more than 2 percent of the words can be hyperlinked for monetization purposes.
c. Hyperlinks should not be formatted or behave differently than other links.
d. Hyperlinks should not be misleading, in either content or placement.
Personally, I never want to see ads, and I've always blocked all ads. But I can empathize with the people who are fine with seeing this kind of ad.