Few subjects elicit as many hostile comments from audiophiles as articles about manufacturers and loaned audio gear. Some posters feel very strongly that ANY loans are immoral and irrevocably bias the review process. I would disagree.
Some manufacturers would like gear returned the moment the review has been finished. To expedite the process, it arrives with return call tags enclosed to make the return even easier. And then there are other manufacturers who NEVER want to see a piece returned after it goes out. As to which are which, that can change from year to year and even, sometimes, month-to-month. Some manufacturers have instituted monthly, or quarterly check-ins on loaners to make sure that their gear is still "needed." I like that.
That word, needed, brings up another point I need to impress upon readers - there has never been one piece of audio gear that has ever been so indispensable to me that when asked for its return I have not immediately acquiesced. In my opinion, there are no audio components made that are so far superior to their competition that they can't be replaced with another piece with a similar function. At my age, I no longer feel any need to buy a review component merely because it needs to be returned. Because I hold any component that I do purchase until it has been discontinued (so I can compare it to its replacement), I have never found selling my no-longer-needed components to be a money-making endeavor, so I avoid it whenever possible.
When a component comes to be reviewed my position is quite clear - it belongs to the manufacturer and will continue to belong to the manufacturer unless A. I purchase it. B. the manufacturer ceases to be a going concern - after no contact for a year I give the gear to charity. C. The gear is discontinued and the manufacturer does not want it returned, it will go to charity. That is my standard policy that I've employed for the last 30+ years.
There are several practical reasons for manufacturers to encourage long-term loans to reviewers. The first reason is simple - if a reviewer uses a component in a review of other gear it will receive additional exposure. Given the cost of a page of advertising, long-term equipment loans are one of the least expensive forms of marketing. For some of the new Asian earphone manufacturers seeding product to "reviewers" in exchange for writing about the product has become an accepted standard methodology for gaining exposure. For some, it's their primary means of marketing.
Another practical reason for review samples to remain out in the world as loaners is that it would cost more for a manufacturer to recall, refurb, and resell the component than any profit recouped from those actions. Take a $500 MSRP earphone. Dealer cost new - $250. Dealer cost for refurb - $150. That doesn't leave much room for any profit for the manufacturer. Wouldn't it make more sense to just leave the sample where it is, wherever that may be? One mention in a future review is worth way more than the meager profit (if there is any at all) of selling the sample as a refurb. Some companies do the math and come to the same conclusion. Obviously, the math for a $500 component is different than a $50,000 one...
So, there you have my own methodology and thoughts on loaners. I don't expect everyone will agree, but since I actually work in the industry I have found a pragmatic approach, always deferring to the manufacturer's wishes as it regards their gear after a review is done, has been the best loaner policy...