It is certainly not free at "hundreds of thousands of requests per minute" scale.
Server load at that scale is measured in single-digit dollars per month, and bandwidth _might_ require two pipes with enough images being loaded. Multiply by 20 for replication and latency issues, and you're still only talking $200/month.
As a ballpark, even bad ad impressions bring in $10/mille, maybe $1/mille for something unintrusive. Does Anandtech get more than 200k impressions per month? If we're talking about a "hundreds of thousands of requests per minute" scale then I would certainly hope so.
Users coming direct from organic search to an article tends to have the best: they’re more likely to be buyers and/or find the ads interesting.
Main downside with ads on tech sites is their users block ads a lot.
Even if you have a server doing something 100k rpm isn't insane amount of traffic to handle. It requires a relatively modest server.