Firstly, the term "practical atheism" expands well beyond people who consider themselves to be atheists or agnostics. Practical atheists may believe in some form of deity, and even intellectually affirm orthodox Christian doctrine, only to manifest a deplorable disconnect between "faith" and lifestyle. In terms of Psalm 10, practical atheists live as if God has no direct bearing on their behavior, presuming that He will not judge them as sinners.
Secondly, even most professing atheists hold to some system of morality. A few, perhaps, are largely amoral, but they generally end up as mass murderers or serial rapists. Practical atheists, on the other hand, generally adhere to some type of moral code, though it's usually external. For instance, they may be physically faithful to their spouses, while seeing nothing wrong with flirting in online chat rooms. They adopt their own moral code, but draw the line at allowing God to impose His morality on them.
Having made these disclaimers, I now want to proceed to the passage:
2 In arrogance the wicked hotly pursue the poor;
let them be caught in the schemes that they have devised.
3 For the wicked boasts of the desires of his soul,
and the one greedy for gain curses and renounces the Lord.
4 In the pride of his face the wicked does not seek him;
all his thoughts are, “There is no God.”
5 His ways prosper at all times;
your judgments are on high, out of his sight;
as for all his foes, he puffs at them.
6 He says in his heart, “I shall not be moved;
throughout all generations I shall not meet adversity.” ~~Psalm 10:2-6 (ESV)
Most of us read passages like this and say (with arrogant sniffs), "Well, that's not me!" No? Perhaps we haven't hurt those who are economically inferior to us, but have we crushed an intellectual opponent for the sheer joy of winning a debate? Have we supported Planned Parenthood, and in so doing snuffed out millions of innocent lives on abortion tables? Have we affirmed homosexuality, robbing people of hope that they could enjoy heterosexual marriages or use their celibacy to honor Christ?
Practical atheists concern themselves with their personal agendas, adjusting behavior to politically correctness (assuring acceptance from their peers). If they understand that they contradict the Lord's holy standards, they salve their consciences by convincing themselves of His non-existence, or at least His indifference. They may marginalize Him subconsciously--in fact, nine times out of ten, they have no idea that their actions essentially declare their atheism.
I pray that the Holy Spirit will keep my heart devoted to the Lord, fearing to sin against Him just for the sake of my fleshy desires and impulses. Whether I like it or not, He sees my darkest thoughts, and holds me accountable.