I work in a company that sells B2B software for developers, and as developers we do sometimes talk directly to our customers, it's a lot faster having a person on each side debugging the problem.
But our software does cost upwards of $5000 per year, so they certainly pay for that level of support.
I do understand why Apple bans people from helping outside of official support channels. Imagine how annoying it would be if you were in support and talking to a customer who is now telling you that an Apple developer somewhere on the internet suggested you try some solution that didn't work or even made things worse.
At my old company, they made devs do 2 hours of support a week, that was frontline support, so we got all the bullshit queries.
It definitely helped resolve some common issues. For instance, we got a lot of support queries about people forgetting their password. There was a reset password option, but it wasn't large on the screen, so customers missed it.
I made a fix, so that if they got their password wrong, there was a nice big message with the forgot password link. We didn't have a single forgotten password query after that. It was a 10 minute fix.
There were a lot of other support queries that had easy fixes that didn't make it back to the devs.