They make them money because:

• they use reddit
• spez gets some nice usage stats to show off
• as a direct consequence, advertisers keep paying to put their ads
• also as a direct consequence, investors’ confidence in reddit continues to recover; there’s a real possibility that, when it IPOs, it will actually go for a decent price

Now, if enough people go commit ad-block, and advertisers somehow become wise to that fact… then maybe it will hurt reddit’s bottom line (at which point spez will start trying to emulate youtube’s anti-ad stuff).

But as it stands, especially if most of reddit’s usage is through reddit’s mobile app… I’m not really sure how you can block ads there.