It's legal in the way that celebrity/professional endorsement is legal in any other field. It's not inherently wrong, and you would expect that an ostensibly effective product would be touted by respected professionals. And just to be clear, when speaking for the product, the doctors generally must stick to the company's slides and official talking points -- though the doctors have final say over what they present [1].

This is again, not necessarily evil -- in fact, it's a logical continuation of the law which mandates that companies only promote their drugs exactly and only for what FDA regulations approved the drugs for. The problem, as presented in the OP, is when doctors unofficially (i.e personally) endorse/promote/heavily prescribe drugs for off-label uses. Again, not an illegal thing on its own, as doctors are trusted to prescribe what a patient needs based on their professional judgment (or else, Dr. "House, M.D." would be in prison)...but the worry is that a financial relationship with a company (i.e. the payments for touting the official line) have the potential to taint a doctor's professional judgment.

[disclosure: I used to work at ProPublica and on the Dollars for Docs project]

[1] http://www.propublica.org/article/double-dip-doctors-paid-to...