Professional writers write. That's their day job. They write, they think about writing, they work out plots in their heads, they think about dialogue and setting and characters, etc.
They also goof off at work, just like the rest of us. Productive days are filled with writing. Unproductive days are filled with websurfing, Tetris, the New York Times, etc. Presumably, they as individuals do as much self-justification about their goofing off as any of the rest of us do.
Successful people generally goof off less than unsuccessful people, but we all have bad days.
I admire people who are productive writers while holding down a demanding day job. By "productive" I don't mean turning out 2K/day of crap. I mean producing work of a quantity, quality and timeliness that will be purchased (if done on spec), that will fulfill contractual obligations and that will inspire, thrill, scare, enrage or seduce readers.
Ultimately, I'm just impressed by someone who can write a steady stream of good fiction while working 40+ hrs/week as a teacher, cop, accountant, systems analyst, puppeteer, caregiver, lawyer, etc. When do you people sleep?