--testnet not the same as --networkid 3

[MitchK]

Not sure if it is an actual issue or if it is intended that way.

System information

Geth version: 1.7.3
OS & Version: Linux (Ubuntu) and OSX

Expected behaviour

--networkid 3 to work the same as --testnet.

Both with --networkid 3 and --testnet, it should create a custom genesis block.

Actual behaviour

Only if I use --testnet, it creates the genesis block.

$ geth --testnet --datadir ./.ethereum
WARN [11-28|14:16:14] No etherbase set and no accounts found as default
INFO [11-28|14:16:14] Starting peer-to-peer node               instance=Geth/v1.7.3-stable/darwin-amd64/go1.9.2
INFO [11-28|14:16:14] Allocated cache and file handles         database=/Users/user/.ethereum/geth/chaindata cache=128 handles=1024
INFO [11-28|14:16:14] Writing custom genesis block
INFO [11-28|14:16:14] Initialised chain configuration          config="{ChainID: 3 Homestead: 0 DAO: <nil> DAOSupport: true EIP150: 0 EIP155: 10 EIP158: 10 Byzantium: 1700000 Engine: ethash}"
INFO [11-28|14:16:14] Disk storage enabled for ethash caches   dir=/Users/user/.ethereum/geth/ethash count=3
INFO [11-28|14:16:14] Disk storage enabled for ethash DAGs     dir=/Users/user/.ethash               count=2
INFO [11-28|14:16:14] Initialising Ethereum protocol           versions="[63 62]" network=3

If I use --networkid 3, it creates a default main-net genesis block.

$ geth --networkid 3 --datadir ./.ethereum
WARN [11-28|14:17:25] No etherbase set and no accounts found as default
INFO [11-28|14:17:25] Starting peer-to-peer node               instance=Geth/v1.7.3-stable/darwin-amd64/go1.9.2
INFO [11-28|14:17:25] Allocated cache and file handles         database=/Users/user/.ethereum/geth/chaindata cache=128 handles=1024
INFO [11-28|14:17:25] Writing default main-net genesis block
INFO [11-28|14:17:25] Initialised chain configuration          config="{ChainID: 1 Homestead: 1150000 DAO: 1920000 DAOSupport: true EIP150: 2463000 EIP155: 2675000 EIP158: 2675000 Byzantium: 4370000 Engine: ethash}"
INFO [11-28|14:17:25] Disk storage enabled for ethash caches   dir=/Users/user/.ethereum/geth/ethash count=3
INFO [11-28|14:17:25] Disk storage enabled for ethash DAGs     dir=/Users/user/.ethash               count=2
INFO [11-28|14:17:25] Initialising Ethereum protocol           versions="[63 62]" network=3

Steps to reproduce the behaviour

Before each start, make sure you have a clear data directory.
On Mac (same issue is on Linux):

brew tap ethereum/ethereum
brew install ethereum
rm -rf ./.ethereum

[stale]

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