There are a great many who have and do. I see nothing contradictory with someone believing that Jesus is their savior and all of that (yes, oversimplifying), and "continuing to sin". That includes ignoring the fact that naturally all believers who don't convert and then immediately die "continue to sin". You can claim that someone who "truly believes would not do so" and thus anyone who does continue to sin never was a "true believer", but then that's merely because that's how you've defined "true believer" to be, rather than actually having anything to do with a result of belief.
Born-again Christians will commit sins after salvation. The Bible says so. However, they won't continue in sin. That means, they won't live in a continuous pattern of sinful life. They have turned away from the direction of sinful life. When they do sin, they will repent, rather than just ignore the sin and do more.
I John 1:
6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: 7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
8 If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. 10 If we say that we have not sinned, we make him a liar, and his word is not in us.
But seriously, I'm using it more in the sense of extrinsic vs intrinsic decision theory. Legalism (as I'm using it) is the system whereby you are handed a set of rules (stone tablets, if you will) and told that those are defined as "right and wrong". The inherent problem with legalism of this sort is easily seen in the following situation:
1) God is mysterious to humans, and we cannot predict his actions.
God has revealed Himself, and His character, thru His Word. We might not know how He will deal with a certain situation but we can trust that however He does, it will be what is best for us (because of #2).
Why would we need to predict His actions? Can you predict what will happen to you in your daily life? You may have a daily routine but it can be turned upside down at any time.
2) God is superior to humans, which is why we must obey his commands.
The Creator
is superior in all ways to the creation and creatures. That's why we can trust Him.
3) God hands you a stone tablet, containing the 11th commandment: Thou Shalt Rape The Virgins.
The Bible is finished. God won't be adding any more to His Word. The time of personal revelation is done.
Also, God doesn't expect a believer to do anything that would contradict His other commandments.
Therefore, you have set up a situation that can't happen.
If the above situation seems unfathomable or impossible to you, then you've either modified that moral framework so that it extends beyond that (in which case it is now subject to your personal control), or you're trying to limit your supposedly omniscient and all-powerful (you left that one out, but I'll assume it was meant to be included) God to merely human feelings, thoughts and emotions.
I didn't list all of God's attributes (there are many more) only for posting time and space. Nothing was intentionally left out. God is definitely all powerful.