Care to provide some documentation for that.
While it is impossible to document the thoughts of individuals unless they reveal those thoughts themselves, actions of the individuals can be an indication.
People are motivated to break the law when they believe they can obtain something valuable for little or no expenditure of their own resources and not face negative consequences. A fair description of an illegal's thought pattern.
A legal will have expended the time, resources and effort to arrive legally and thus has already paid his part so, obviously, a legal does not think everything is free. Occam's razor indicates an illegal holds the opposite perception of a legal.
Totally wrong
People break the law for all kinds of reasons. Some do it out of conscience (christians in rome, tianemin square, the underground railroad in pre civil war US). Some do it for material gain. Some do it for sexual gratification. Some do it for power. Some do it for revenge. Some do it just to do it (vandalism).
Second, the illegals do expend a lot of resources (they come from many miles, leave their families and often work when they get here).