People do what they feel they can get away with. For some, it stops at going a little faster than a government decided speed limit, or downloading a song on the internet knowing it doesn't really hurt the earnings of the artist. Everyone though has that grey area between right and wrong, and denying its existence entirely is colosally stupid. We take what we want, when we want it, justification is only necessary when the choice far outweighs your internal moral compass. For some, sociopaths mostly, their grey area is far larger. They will always game the system, take more, give less.
The greatest example of this thought process is the demand of paying more in taxes, but making no effort to provide that money absent from force. "I can afford to give more!" is laughable in that nobody does it voluntarily. If charitable donations were not tax deductible, they would plummet. People only willing give up until they see an effect on their own comfort. It's fundamental.