This has annoyed me A LOT quite some time now. I think site owner tries to make more money by allowing this kind of scripted spamming. I really hate this because important links in postings are lost because people won't click any links anymore because of these frigging spam links.
When you say it does happen only on certain browsers or machines, I don't think it's made that way. I believe it is intented to happen with all setups, but some setups just manage to block it.
For me it happens for example with OWB on MorphOS and I don't believe my machine is infected with any adware add-ons.
This is over the limit I'm willing to accept for advertisements.