There is an alternative motive for the Democrats not to enforce the law, and that is these illegals families members who are legal will support them come election time. The Hispanic vote outside of Florida is very democratic. (In Florida, Cubans usually vote Republican)
Quite so, Hispanics are natural Democratic voters, and they vote Democratic by a margin of 2 to 1 (and I understand the margin is increasing, more and more Hispanics are voting democratic). So no doubt Democrats have reasons not to enforce immigration laws.
However, so do Republicans. Big business can get cheap labour from Mexico which increases their profit. Instead of paying say 10$ per hour to a legal, they can get away by paying 5 $ an hour to an illegal. Since big business is a natural constituency of Republican Party, Republicans also are not inclined to enforce immigration laws. That is why Republicans did nothing when they were in power in Washington for 12 years.
But Arizona Republicans found a happy way out. Their bill targets only Hispanics (democratic constituency), while leaving the big business alone (the Republican constituency).
The bill is politically motivated (and of course, racist and xenophobic).