I think most of the photographers that work for the paper also earn income through other avenues. Unless a photojournalist is placed with a military unit overseas, I can't see them earning a steady salary and benefits.
Would we say that it is more convenient and cost-effective than hiring in-house professionals? As Paul Stevens says, freelancing allows a photojournalist to handle several jobs simultaneously, hence the possibility to earn more.
Does it apply to all print media companies or those with the financial muscle? Hiring freelancers may not be cheaper than having in-house staff. There are instances when both categories of professionals exist in an organization.
Larger news agencies would probably not rely too much on freelancers to maintain a certain standard. Photojournalism aims to deliver a message, and some organizations like their employees to have the same values.