They address this is a way to cut down on spam.

    A crucial difference between Urbit and other networks is that planets are
    scarce. Even when the network is fully populated, there are only 4 billion.
    Early in Urbit's life, most stars and galaxies are not yet operating, so
    far fewer are available. No one will ever be able to get planets trivially
    and for free.

    Urbit is a friendly network: a network on which you can assume that a
    stranger is nice until proven nasty. Friendliness is a direct consequence
    of scarce, individually owned identities. We're not changing human nature,
    just creating the right economic incentives.

    Most forms of network abuse are "Sybil attacks": they rely on an infinite
    supply of fresh identities. Scarcity makes reputation work. Spam is a
    business; if the cost of a new planet exceeds the amount of money you can
    make by spamming from that planet until its reputation is trashed, there
    will be no spam.

    Shady stars and galaxies that sell blocks of planets to spammers will also
    develop reputations as "bad neighborhoods," damaging the value of the whole
    block. Abuse at any level is designed to be counterproductive and
    economically self-terminating.

    Urbit has no reputation system at all at the moment, simply because we're
    so small that friendliness is automatic. The clear and rigorous structure
    of the address space is not a reputation system; it is a platform on which
    any number of such systems can and should be built. But we can't build one
    until we need one.