Part of me really wants this kind of information to be recorded somewhere and made public some number of decades into the future. It would be absolutely fascinating as a student (in the broader sense) of technology and business to read all the confidential agreements, product placements, and back-room deals that facilitated the outcomes of technological history, even if it's after a very long delay.
By any practical measure Apple is indeed paying in that they gift the show with free iPads / iPhones / Computers / etc. Hence why the credits usually say that "promotional considerations" were provided by Apple.
However as others have said, Apple appearances do tend to be much more subtle, insofar as a big Apple logo is subtle.
In those cases, Apple is clearly not paying to have their merch being used and the show clearly didn't get approval to "officially" use the Apple logo which is why they had to cover it. But, in the end, it is still clear that this thing they use is an iMac.