I understood what you are saying but you seems to be under a misconception about what it is a firewall does
a computer can offer services and inputs from other computers across a network those can be things like ftp, web server mail server or sql server or....
if such features are turned on and one have no firewall everybody can access them unless of cause the ports they rely on are blocked by the carrier
ios devices offer no such services so there is no way in for people on mistrusted wifi hotspots or ..
even the script kiddies denail of service attacks would not because there are no services to attempt to access
so basicly having a firewall would be like you never connecting your computer to the internet and never using external media and yet feel you needed a virus scanner