I saw Rasmus many years ago (think PHP V 3) and he was an excellent speaker. Most conferences will pay the speakers to speak, so if they can get someone to cover travel and hotel, they can at least break even, even make a bit of scratch. I met some of the PostgreSQL crew at an OSCON event a couple of years ago, and I know that their companies had paid for them to go respectively. I think part of it has to do with keeping your people happy. If Jeremy gets to go to OSCON, Jeremy is happy, if Jeremy is happy, he'll stay at Yahoo.
It was funny, we (Tom Lane and Bruce Momjian the postgresql guys) went to dinner with a whole table full of MySQL guys. It's funny that when it's dinner and just hanging out, the animosity just sorta dissapears and we all have dinner and chat. MySQL AB did pick up the whole tab though, so that was pretty cool of them. I was VERY impressed with Tom Lane. One of the smartest people I've ever met. Bruce is also quite smart, but Tom is freaky smart. About 2/3 of the MySQL folks were tech support types or CEO types, but a couple of their actual factual developers were there, and so was Jeremy, if memory serves.
I'm guessing that most companies do it to keep their people happy, and to also attract users and or other coders for their projects and such. I'd highly recommend any to go to a few cons and get to meet some of the "heroes of open source" that show up.
I still remember what Rasmus said: "I really like Java, it's a great language. I just wish it worked."