During the Steve Scott missions, Tommy is dead-set against featuring a giant shark in Scott's "movie" Bite, claiming that it wouldn't be successful. Then you realize that Tommy's been behind bars from 1971 to 1986, so he missed the release of a certain wildly successful 1975 movie featuring a giant shark, or at least the GTA equivalent. You can even hear him say "Who wants to watch a movie about a fish?" just after the first Porn Studio cutscene.
On VCPR, Congressmen Alex Shrub reveals that the government keeps prison costs down by letting criminals go free. The game begins with Tommy getting out of the jail, and he keeps getting off whenever he's arrested in-game.
The Prologue states that Tommy served fifteen years for killing about ten people, while during actual gameplay, murdering over half the city with a military helicopter that you stole can be overturned with a measly one hundred dollars. Easier to sleep at night by taking into account Gameplay and Story Segregation.
That, and the laws were potentially a LOT different fifteen years ago - since the earliest we see of GTA's Crapsack World is The '80s, perhaps law enforcement was actually somewhat stricter back in The '70s.