Because when you actually read it, you will download 1 article per 30 minutes. If you batch-download all of their content now, you'll create much more peak load and, hence, costs for them.
You can use --limit-rate=500k if you want to limit the bandwidth you're consuming.
And load does not translate to cost for everybody. If you saturate the connection to my VPS, I don't pay more, it just gets slower for everybody in contention. I can spin up mirrors but if I'm offering a free resource like this, I'd be more likely to limit the bandwidth-per-client-IP or just actively let it run slow. They could even limit the bandwith to the subdirectory with...
On the other hand, if you download it all at once you aren't constantly reloading a page when you get back to it and you aren't limited to reading it when you're online. I do see the point you're making, though I think it depends on the author's perspective.
