What exactly is the dangers of talking about sourcing on the internet?

[Sempiternity]

I know across the internet talking about sourcing is a big no-no, but what specifically about talking about sourcing online can get you into trouble? What if a website or person told someone, in a country where it is legal to buy and own, how to legally get a substance, could that also have legal ramifications? I'm most definitely not asking about this site doing as such, more asking in general.