alt Hacker NewsSoftware license ≠ terms of service.
They can give you the right to reuse their source code for building your own service using your own network (an aspire to give you this right), but still forbid you to connect to their service on their network with third party clients.
In practice, they don't seem to enforce that a lot (anymore).
It's unlikely that their ToS would be enforceable in court. The fact that Apple never tried to sue Beeper speaks to this.