A Jack Rasta is somewhat like a Jack Mormon, a person who strays from their faith for various reasons.
Of course, in the United States, very few people are raised in Rastafarian families, so most Jack Rasta's are people who wish to adopt certain features of the Rastafarian lifestyle, such as the good vibes, the ganga and the dreadlocks, while not having to obey such rules as Veganism, abstinence from alcohol and a simple, non-violent lifestyle.
Since very few Rastafarians are able to always keep all the precepts of Rastafarianism, the judgement of who exactly is or isn't a Jack Rasta is a subjective one. Sincerity probably has something to do with it, a person who really believes in peace and love yet can't give up milk chocolate is probably not a Jack Rasta. Someone who likes to get drunk and sing along with Bob Marley is probably a Jack Rasta.
Jack Rastas are a noticeable part of the scenery in many parts of our country.