Does 802.3af compliance require support for both Alternative A and Alternative B?
Th spec requires all powered devices (PD) to be able to be powered from both alternatives.

This way, the power sourcing equipment (PSE) can freely choose which way it wants to provide power. The reason is that, indeed, simple PoE injectors (called mid-span) can then provide power on the unused pairs to simplify their design. And full-blown ethernet switches that act as PSE can provide power on the signal pairs, to eventually reduce wiring requirements.

So you're fine, as long as the camera claims 802.3af compliance. However, if it doesn't claim full 802.3af compliance and explicitly asks power to be delivered on the data pairs, you may want a clarification from the manufacturer.