Notice it says the agent is open source, not the whole thing.
The software running on your machines is open source. It can be reviewed. Quite important for people not wishing to run closed source software on their machines.
Yea, if you're hosting with them, trusting them is handy.
This Longview agent can be run anywhere though, doesn't have to be on a Linode machine. Not entirely sure why you'd want this setup, but still..