Regarding the illegality of portals, it’s actually from in game books. Basically the Kirin Tor don’t want people misusing portals (causing mages to kill themselves by teleporting into walls/chairs/eachother) or by harming others in the same way. The ‘designated drop off points’ are ultimately just lore built by game design, I think, since you can apparently get around it by having a certain level of license or just not giving a shit about the Kirin Tor’s laws. It isn’t about ley line strength.