Banks offer sign-up bonus in order to bring more customers and they hope that after a year these customers will stay and start paying annual fee and keep using the card.
I'm sure Apple feels they will get huge interest in their card even without a signup bonus, so there's no need to offer it.
Right - it's a confusing strategy IMO. In general wealthier people who probably don't revolve credit (e.g. pay in full each month) want premium rewards, which this doesn't offer at all, so my hunch is that this will appear to more middle income/lower income individuals who pay interest (cost of interest negate any rewards), which is a bit strange since the branding/marketing and Apple (+ Goldman Sachs) generally target wealthier individuals.
I'm sure Apple feels they will get huge interest in their card even without a signup bonus, so there's no need to offer it.