Sin is much greater a problem than Calvinism believes, and God is more holy a God than their doctrine will allow. There is no getting around that when you view Calvinism alongside a traditional form of the faith. Calvinism is simply "easy believism".
Are we talking about the same Calvinists as the Puritans? The same ones who said to claim to be born again and to continue in any known sin is an impossibility. The ones who say that the only way men can on their own provide just recompense to God for the smallest sin, which an infinite offense against a Holy God, is to suffer endlessly? The ones who have been ridiculed for hundreds of years now for living such exact and attempted holy lives?
Regarding the verses in 1John, do you know what Owen wrote about the purpose and order of the gospel as meant in those verses?
"And as the whole effect of the doctrine of the gospel in holiness and obedience consists in the soul's being cast into the frame and mold of it, Romans 6:17; so the whole of the apostasy from the gospel is principally the casting of the into the mould of this false reasoning,
that sin may be indulged unto upon the account of grace and pardon." In other words, he complains that people tend to view the propitiation available when they sin while forgetting that the first point was "that we sin not". (From John Owen's works book 19 Indwelling Sin in Believers, chapter 12 of 17 chapters of the main man who leads the sect that doesn't take sin seriously enough)
Of all the statements I have ever heard anyone make on here, the idea that the Calvinists don't take sin seriously is the most ill informed and ignorant yet.