Yes, I feel as though they should turn on LFG automatically. Maybe even once you hit say lvl 10 or so, you auto-join it.
However, the people here saying they’re going to RCR everyone who advertises in trade are being a bit dramatic. Reporting is a bit extreme. If you don’t want them in trade, then tell them(and others on your server) to use /join lookingforgroup .
While I never played in vanilla, in TBC/WOTLK before auto queues and such, I remember people posting in trade all the time for groups, and I don’t remember anyone ever being upset about it. As long as they aren’t spamming or posting across multiple channels.
Since Blizzard doesn’t auto-force people into the LFG by default, it’s a little too much to report people for not using it.