Why do you think that should be suspicious? Most open source software comes with multiple installation options: some faster ones if you only need to use it as is (installation script), some more flexible ones if you intend to modify it (build from source).
They don't tell people not to build from source. They don't recommend it. They don't recommend that you don't they just aren't saying it's the blessed way to get up and running. They have a whole "Developers" document about it.
Why do you think that should be suspicious? Most open source software comes with multiple installation options: some faster ones if you only need to use it as is (installation script), some more flexible ones if you intend to modify it (build from source).