It's not that simple, 'erm. They can't (well, they can if they want, I guess) stop your phone from running all other apps 'cos then you wouldn't be able to make/receive phone calls, use your sat nav, use your web browser, use your CC app, use your share dealing apps, etc, etc, etc. They need to prevent/override what the app does e.g. by putting a log out button where the accept button appears when a job is offered or logging you out if you tap the screen more than a predetermined number of times. I think their app puts you busy when you activate a fake location app... a similar approach would be required for this.
PP probably migrated from using addons such as adobe flash because some clever folk figured out how to hack websites with it so their use database may have been at risk.