The #2 promotion showing fighters from the #1 promotion attending their event helps promote and validate Bellator as a show worth watching. It doesn't matter if they were actually there to say hi to Scott or root on a teammate.
It's all about perception. I am willing to bet most, if not all, MMA fans who watch Bellator know who Cormier, Rockhold and Kos are. When recognizable UFC fighters are seen attending a Bellator event, it gives off the vibe that Bellator must be a hot ticket.
Same reason celebs who can't even name what the main event is will get camera time on a UFC PPV.
The reason "uncle Dana wouldn't show Bellator fighters sitting ringside at a UFC event" is because the UFC has absolutely nothing to gain by showing Bellator fighters attending a UFC card. All it would do is name-drop and give camera time to a bunch of fighters that most their fans probably aren't familiar with.
"Emmanuel Newton and Blagoi Ivanov enjoying the UFC fights tonight!"
OK...who the fuck is Emmanuel Newton or Blagoi Ivanov to the average UFC fan? How does the UFC benefit at all from acknowledging them on the air?