It's a pretty important legal difference. The administration is now trying to treat it as a criminal offense, trying to enforce it as a criminal offense, using that to justify calling huge swathes of immigrants 'criminals', etc.

Consider this scenario. A friend of yours walks up to you and says "man, I got a ticket yesterday." You will have a general idea of the level of enforcement involved, how that's looked at by the government and others, etc. You probably do not see your friend as a criminal.

Now, suppose the same friend walked up and says "man, I got arrested yesterday, I just got bailed out." You will have different ideas of the enforcement model and perceptions of that. There is now a greater probability that you see your friend as a criminal.

Because when Trump said “we are going to deport the worst of the worst” people took that to mean criminal gangbangers. Then all of a sudden he’s deporting hairdressers, students, and abuelas and people are understandably surprised Pikachu, saying that’s not what they voted for.

It’s a sleight of hand, they say “the worst of the worst” and what they mean is “all immigrants (not just the illegal ones) are the worst of the worst”

This is why they’re going to lose their majorities next week.