The first time you run swarm, you need to specify a --bzzaccount.

By the second time, this should no longer be necessary.
I suggest:

• Whenever there is a key in the swarm data dir, the swarm command should use that one.
• If there are several keys in the bzz datadir, the client should give a helpful error message like "Multiple bzz accounts found a, b, c, d, please specify which account to use with --bzzaccount ADDRESS" or sth like that.
• If there are no bzz accounts in the swarm datadir, but there are keys in geth keystore, then swarm should offer the keys in the geth keystore as a suggestion: "You must specify an ethereum account to use as your swarm id with --bzzaccount. Ethereum accounts found: a, b, c, d"