The "left seat" occupancy by the pilot (in command) of - transport planes - is sometimes mandated by (some) airline policies, but as far as regulations, by accessibility of some controls which are only accessible from the LH
As an example the steering of the nose wheel on the 707 was originally on the LH
side of the pilot (in command) station... however, some airlines, i.e. Qantas, ordered their 707-338Bs to be equipped with a tiller on the RH
side as well, so the co pilot could steer the airplane. Here in Argentina, a captain can sit in the left or in the right hand seat if he so wishes... no restrictions, the 747 has a tiller on both sides...