Exposure/free advertising in (primarily Swedish) tech-media, would be my guess. It may also encourage people to choose their services for other future hosting-related services.

I think it's mostly a brand awareness/marketing type of trade off. You get your pi hosted for free, and then you are impressed with their uptime, service, whatever, and you are more likely to recommend your friends use this company's services, or get a paid vps or something from them yourself next time you need one. I took advantage of edis's offer of free pi colocation in the past, and since then I've also purchased a vps from them, and had several friends do that as well.

People want to see if there's a market of tiny hardware hosting, and if yes, be first to develop necessary hosting technology (e.g. power supplies, relay boards for remote reboot, optimal RPi mounting on the server blades etc).

It's a loss leader, host the Pi for free, then sell the reboots, SD card swaps, extra bandwidth, IP blocks....

Eventually upsell the customer to a full hosting package if they had used the Pi for an extremely cheap hobbiest/project/dev environment before going to a real server.