In journalism running into conflicts of interest seems to be a recognized "part of the job" for reporters and news gatherers. It's a bullet point in the Society of Professional Journalists' Code of Ethics, implying that any journalist who wishes to be taken seriously should know about it. So, reading the code of ethics lets journalists know a few things that should be common sense: Never pay for stories, admit to your audience any possible bias in your writing and refuse preferential treatment from sources. Despite all that seemingly common knowledge, it keeps happening.
I wasn't fully familiar with the practice of "checkbook journalism" until reading the article on it by the Columbia Journalism Review. It's a practice that seems obviously wrong to many journalists, but they participate in it. I believe the amount and weight of outside pressures makes people more likely to give in to these morally questionable practices. To put it bluntly, finding money in the journalism field is difficult these days. The number of printed magazines and newspapers is dwindling as the medium is in a transitional period between switching to digital, while still trying to keep print journalism relevant with varying degrees of success. The Sun might not be considered a beacon of thoughtful, award-winning journalism these days, but it is popular and one could probably live comfortably while working there.