For a possible clue to why Marco Rubio does what he does, these were the top 10 donors to his 2010 senatorial campaign (link):

In 2010, no other candidate got more money (directly) from the Koch family than Rubio. As discussed at that link, they support organizations that advocate for loose borders. And as discussed in depth on the Marco Rubio page, he supports a massive legalization program for illegal aliens. While a direct link between him receiving their money and him promoting amnesty isn't known, t's extremely unlikely that he would have received any money from the Koch brothers if his immigration position were that of, say, Pat Buchanan.

The first organization in the list (known around here as the "Club for Profits at Any Price") doesn't appear to have taken a position on immigration, but they can be assumed to be part of the high immigration right. Their goal is to help businesses increase their profits, no matter the costs borne by everyone else. Their Chris Chocola did want amnesty author Mike Pence to run for president.

MCM Corp. is a heavy construction outfit; mass immigration would increase their bottom line by lowering their labor costs.

Flo-Sun Inc produce sugar and is run by the Fanjul family; per this "[they] might be considered the First Family of Corporate Welfare". They were mentioned in Christopher Hitchens' "No One Left to Lie To: The Triangulations of William Jefferson Clinton" as recipients of racial set asides despite being quite rich.

Crow Holdings manages real estate as part of the trust of Trammell Crow; whether they'd be beneficiaries of increased immigration isn't known.

Bank of America is, like other banks, a strong supporter of massive and illegal immigration. See immigration banks.

More on Rubio's funders here (March 2011):

Sen. Marco Rubio (R-Fla.) has made no secret of his pro-business agenda since taking office in January. It may come as little surprise, then, that his campaign was backed largely by the industries he now supports. Data obtained by the Center for Responsive Politics from the Federal Elections Commission shows that Rubio was the largest recipient of Koch Industries campaign money for U.S. Senate races. He also received more money than anyone running for national office outside of Kansas, where Koch Industries is headquartered. Rubio’s state of Florida is home to a major Georgia-Pacific paper mill (Georgia-Pacific is a subsidiary of Koch Industries) that has been at the center of a major battle between Koch industries and environmental regulators.

Many of Rubio’s other campaign donors have connections to the Koch brothers. Some are more closely affiliated with the Kochs than others, but all those with ties are plugged in to the national network of business leaders and anti-tax and anti-regulation advocates that Charles and David Koch have endeavored to build through conferences and think tanks.

The largest single contributor to Rubio’s campaign by far was the economic libertarian organization Club for Growth, whose members gave $346,450. Club for Growth has had ties to the Kochs since its founding in 1999, the closest of which being that one of its directors, Howard Rich, is also a director of the Cato Institute, a think tank that Charles Koch provided the initial funding for and on whose board of directors David Koch now sits.

Elliott Management, the Senate Conservatives Fund and Flo-Sun, Inc. round out the list of the top five contributors to Rubio’s campaign (Club for Growth’s number one, Koch Industries is number five). Elliott Management is a hedge fund management company run by Paul Singer, an investment banker who chairs the Manhattan Institute, a conservative think tank underwritten in part by the Koch Family Foundation and its affiliates. Singer has helped emcee controversial Manhattan Institute events that Clarence Thomas, Samuel Alito, and Antonin Scalia have attended in recent years.

The Senate Conservatives Fund is a campaign fund started by Sen. Jim DeMint (R-S.C.), who has attended Koch-sponsored events and was given the Washington Award by David Koch in 2009 for “defending the American dream.” Koch Industries is one of the top contributors to the Senate Conservatives Fund PAC.

Flo-Sun is a sugar and real estate conglomerate owned by the Fanjul family of southern Florida. Flo-Sun, which owns Domino Sugar, has been active in fighting the EPA over water pollution regulations in the Everglades. The Fanjuls are family friends of the Kochs; David Koch and his wife Julia traveled to the Dominican Republic in 2009 for the wedding of family scion Christina Fanjul.

Rubio’s connections with the vast Koch Industries web of course did not end with his election. The senator’s chief of staff, Cesar Conda, was once an aide to Dick Cheney and is said to be one of the architects of the Bush tax cuts. He is the former executive director of the Alexis de Tocqueville Institution, a libertarian think tank funded in part by Koch-run groups such as the Claude R. Lambe Charitable Foundation.