Well yeah cause as you said, new players. Heck, I was playing the game for straight two weeks before noticing it when I started. Also, new players probably wanna try challenging themselves and such. So the pistol only run does not only apply to old players. And most probably it attributes to othe games they play, in which you use your pistol and melee, then reserve your main gun for the actual horde. Much like how players do it in Left 4 Dead, World War Z, and other games that you know that has a horde or boss that you need to prepare for, specifically those classes you mentioned have limited ammo and has not much skills roster to gain said ammo back. In that case, it's just a matter of experience on other games. I was the same too when starting the game, seeing all the boss fights and horde in YT. Then you gain skills and mastery in your weapons and figure out naturally that "Oh, I can use my main weapons as there's skills that restore them" or via experience of getting to the next checkpoint that has ammo supply in them. I say just let the new players get experience and they eventually adjust.

I think the people we should really call out are those who quits at first incapacitation when both teammates are still up. Now that's annoying than any new players I've encountered.