Some Trump supporters are pissed the President got so little airtime at the NBA Finals compared to stars at MSG -- and it turns out that wasn't an accident.

Here's the deal ... DT made history as the first sitting prez to attend the Finals last night -- but was only shown on ESPN twice ... and one of those times he was mercilessly booed. Meanwhile, celebs like Ben Stiller, Spike Lee, Tracy Morgan, Timothée Chalamet, Tina Fey, Fat Joe, and Larry David got plenty of network love.

ESPN sources tell TMZ ... Trump got due attention for this historical moment ... but he was also in a bulletproof box -- which ain't as easy to film as all the stars sitting courtside.

And we're told even if some of those stars have been critical of Trump -- they are also die-hard fans and fixtures at big games, so they're getting more exposure. Bottom line -- it wasn't political!