<p>The Department of Labor has standards for unpaid internships (otherwise the employer is responsible for wages under the labor laws), which you can find on the DOL website.</p>
<p>Law firms, engineering firms and similar companies may want to have the student be a compensated employee in order to ensure that any tasks performed by the student are covered under the applicable liability insurance. They will also want to ensure that the student is bound by ethical obligations and internal policies and procedures that they’ve written as being applicable to employees. Sometimes they just don’t want to bother complying with DOL guidelines, and would prefer to pay a wage.</p>