[Obi_Juan_Kenobi]

They can take either; waiting for confirmations is only a matter of reducing double-spend risk.

In practice, seconds after a transaction has been signed and broadcast, it is already very unlikely to double-spend. Miner incentives are such that the first-seen transaction is the most likely to be used. A delay of a couple seconds is sufficient to account for network-propagation lag.

You wouldn't do this for high-value transactions, but there's some threshold where real-world risk is lower than the convenience of a fast transaction, and that threshold is reasonably high.