Is enemy optimization poor DMing?

[Sith_Happens]

Mostly likely meleeing with the beholders bite attack.

Which it can do in addition to firing 3-9 eye rays, not instead of.

[Milodiah]

At this point I wouldn't be surprised if the Beholder picked up and used a light crossbow for the duration of the combat...there's "going easy on the players because you don't want a TPK but still set up a situation where one could should happen", and then there's this.

I would have left the table as soon as I noticed this happening, and I'll be damned if I care about the guy talking about me behind my back as if I'm somehow in the wrong. If the guy manages to construe it that way, then either he's a terrifying liar or in the company of people I'd be just as unhappy sharing a table with.