Few unbelievers consider that God takes any notice of their thoughts. They view their own minds as private territory and feel free to let anything they like pass through them.
‘But the words of the pure are pleasant’, or as some render it, ‘words of pleasantness (or delightfulness) are pure’, making ‘pure’ the predicate not the subject. The Hebrew seems to contrast thoughts with words, wicked with pleasantness, and abomination with pure, but the KJV arrives at its translation by supplying the phrase ‘the words of’ borrowed from the end of the verse, and turning ‘pure’ into a description of the man rather than of his words.