I get the feeling it's akin to why GAIA doesn't allow it - people might be under age and unable to. It goes against TOS in the light of "it takes money away from the actual site" since the currency doesn't pass through them. IE Commissions on GAIA for gold/platinum is 100% okay because it's a free currency you earn, and technically you aren't making any money off it.
I wholeheartedly agree and would rather see a thread to see people's commissions to participate with the community here more - but I think they're wanting this forum to be about free unpaid collaboration, which is both good and unhealthy. On the one hand, people can create together and become friends and bond, and it's pretty cool. On the other, more disgusting hand, it keeps that circle of art doesn't pay (unless you're super famous so lol) mentality alive and healthy. 
On note, GAIA people are able to circumvent this system with the loophole of something like: I do commissions for free currency (the gold, platinum, trade for items, or just free) and then they link to their website where somewhere on it says they take commissions. If they straight up say "for RLC email me", that's more likely to be flagged and taken down. And people are drawn by prices vs quality usually. So just posting to their tumblr, where there's a link at the top that says commissions is safer for them; posting a link to the page about the commissions is more likely to be flagged. Again, because of the Under 18/ Under 13 nature of that site.