You have to take all scripture into consideration in order to get the full picture. Someone with one verse might say, "It's blue." While someone with another scripture might say, "It's yellow." When actually it's green- a combination of both. There's a difference between visiting sin and living in it. If someone is completely never going to sin after baptism, then they were born full grown. What baby, spiritual or physical, is born with no growing to do?
The bible talks about growing in Christ, doesn't that imply changing your ways over time? You're right, and the bible is right, someone in Christ WILL quit sinning. I used to smoke for years, now I do not. I grew in Christ. But what if, while I was still trying to quit, someone judged me as not being a Christian? How can I grow in Christ to the point of not smoking if I wasn't in Christ? Do I still have more growing to do? Yes, and I'm trying- and that is the difference- I'm not living in sin, I'm living with the struggle to get out of sin. Meanwhile, where I fail grace succeeds.
Do some people take advantage of God's grace and think of it as a license to sin? Sure they do, the bible says so. But they are deceiving themselves, they are then living to sin. But God knows the thoughts and intentions of the heart. Not that you are judge, but you can't see growth in a person since yesterday, but if you see no growth after a decade, it would appear that they are living for sin instead of fr God, but still, only God knows for sure. Paul was killing Christians, and it probably appeared to them that he would not go to heaven.
And even after baptism he said, "The good I want to do I do not do, but the bad I do not want to do, that is what I keep doing." Someone could have judged him when he sinned after baptism. But take a close look at his intentions in what he said. He said he WANTS to do good but fails sometimes. That implies trying, which brings forth growth, he is still in Christ and covered under grace. But we know that whatever doesn't grow dies.
One of the fruits of the Spirit is patience. Why would God be patient with sinners before baptism, and not be patient with His children after baptism, and then tell us to be good to others, ESPECIALLY to those who belong to the household of faith? We are not to take advantage of grace, but what purpose is grace if completely unneeded or unused?