The reason for the rule is to prevent inter-form hostilities. We've had several incidents where someone went on another forum and said something nasty, which then gave us an influx of even nastier people. EHellDame and the mods don't like that for obvious reasons.
I've never seen anyone moderated for linking something pleasant like a craft site. There are some people who have links to their Etsy shops in their signature lines, IIRC.
Use common sense. If the link is going to bring bad things to EHell, then don't do it!