In a lot of cases, like the one here, it's because they're publishing for a conference or journal that takes submissions as PDF, not for the web. Then they just put the same PDF on the internet.
Publishing to PDF isn't necessarily a bad thing (e.g. would you rather they published to .odf or .doc?). But if you are planning to publish something to the web, you should probably do it in HTML and PDF (I'm assuming that these are all in PDF because they are being published elsewhere, outside of the web as well).
I wonder what most people are using to generate these PDFs? If they are using LaTeX (or just TeX) it should be easy for them to churn out both formats.
