Unless it has a specific 'hill hold' function, possibly on the menu, then as Rob Cowell says it's quite normal with a standard torque converter auto gearbox, far as i'm aware Toyota haven't got round to spoiling these vehicles, yet, with a pointless (expensive to fix and difficult to service) electric parking brake so unless there's some electrickery programmed into the footbrake itself.
If you select low range you might find that tickover revs on the same gradient might have enough torque to prevent roll back.
Could be worth checking the gearbox oil level and condition of the oil anyway, too many seem to believe the blurb that these boxes don't need periodic oil changes just like every gearbox ever made.
As an aside for years Subaru offered a simple hill hold device being little more than a ball bearing in a brake pipe fitting, but they arn't dedicated off roaders as such designed to tackle severe full articulation terrain and i suspect such a simple device would prove to be a nuisance in serious off road situations.