While martial arts knowledge (or firearms, or knives, etc) can be used to hurt someone else, it's not the knowledge in question that is dangerous. Instead, it's the person. Instead, it's the person who is dangerous.
I do not trust felons (and other serious offenders) to carry firearms (it's illegal), and in the same manner, I do not trust them to not misuse such martial arts knowledge.
As Whitedragon stated, in general, serious offenders are incorrigible, and cannot be redeemed. They become repeat offenders.
Does this mean that all serious offenders are incorrigible? No. Of course not. However, it's still a business, and you have to play the percentages. The last thing *any* instructor and / or school owner wants, is to have his pupil's mug shot plastered on the television.
If such serious offenders really want to learn, then I'll suggest this: take your redemption opportunity seriously (not likely), do many truly outstanding things for the community (not likely), get an expungement for your offenses (even more unlikely), and stay clean. Then I'll maybe re-consider.