Also note that though these markets are "free" they are still extremely highly regulated. I don't know much about the Finnish market but I've built trading algorithms for CAISO and PJM, and it is by no means a free-for-all. The rules about what types of entities can participate in which aspects of the market are quite strict and detailed.

Especially post-ENRON there is a strong focus on avoiding even any possibility of appearance of market manipulation. Basically the guidance from our lawyers was: every trade needs to have a strong, explicable economic rationale (apart from the market.) In our case we didn't even want to use neural-network based trading algorithms, since they (at the time) weren't sufficiently explicable for us to have a "rationale" for a trade that might later be called into question.