Idle question about aircraft in movies

Although I'm not a pilot, I find it fun to jot down the tail numbers
of airplanes I see in movies and TV shows, then look them up to see
what kinds of planes they are and where they're based. (Or at least,
where their owners are based. Airplanes are notoriously peripatetic. ;-))

Sometimes the plane's usual company logo is replaced by a logo that's
appropriate to the show in which it's appearing. E.g., in "Batman
Begins," there's a Gulfstream that has a Wayne Enterprises logo.

I assume that these days, those logo replacements are all being done in
post-production. After all, that's a lot easier than actually repainting
the plane! (I guess they could use those temporary decals, but again, that
seems like more trouble and expense than doing it digitally.)

My question is, while the computer jockeys are fiddling with the livery,
why do they bother leaving in the tail number? Just to make the plane
look more authentic? Contractual agreement with the owners to publicize
their plane? Anyone know?


Patty