According to the Tobacco Master Settlement Agreement, 1998, tobacco companies cannot pay for tobacco brand placement in movies.
Additionally, there is a history of litigation for “negligent advertising”:
Negligent advertising - the tobacco companies failed to warn consumers of the risks of smoking cigarettes
By my non-lawyer reasoning, there appears to be certain risks in accidentally portraying a movie as somehow advertising a tobacco product. Because it would be an accident, they have no way of being completely sure that they did not make a mistake. But they can include a disclaimer saying they did not get paid for any tobacco advertising. If they did not get paid, there was no advertising contract because there was no consideration (a legal term indicating that something of value, like money, exchanges hands in order to have a legal contract).
There even has been research associated with trends in showing tobacco products in movies as a result of the Master Settlement Agreement.
There has been a marked decline in the number of movies with TBAs [tobacco brand appearances] released after the MSA [Master Settlement Agreement]. However, the greatest absolute decrease occurred in R-rated movies, and the decrease seen in movies rated for adolescent audiences was not statistically significant.