Not to minimize the 2016 or 2020 elections, which a lot of sources say there was not a level playing field in the DNC, but this year there is an incumbent president. This is how incumbent presidents are always treated. It’s normal and fair and strategically sound.
The same thing happened when Donald Trump was incumbent and nobody made a fuss.
Edit for clarity:
normal - The incumbent candidate has preferential treatment within the party in every election cycle. There are various ways that this manifests, and is usually different depending on the exact circumstances. If one chose, they could drill down into specific details to make it seem exceptional e.g. “It’s never been done in with this specific mechanism or in this particular state.”
fair - If you want access to preferential treatment, become President. The President is the figurehead not only of the country, but arguably even more so of their party. It would be unfair for the party leadership to undermine them while in office.
strategically sound - Incumbent candidates win elections. There is something like a 65% advantage to incumbency. Moreover, a party has limited political, social, and financial capital. If they spend that capital in the primary race, then they start the general election at a disadvantage. There is evidence (and common wisdom) that a primary race actually generates more capital, but I’ve never heard any credible suggestion that it could be a net gain in any area. Running a primary means a less unified party, less financial resources, less voter confidence in the victor.