I think I can take a good stab at it based on what I know and have read here:
1) As stated, the DIS (with narrowly controlled exceptions like DVC rentals) has always prohibited the commercial use of the DIS by its members. You can't sell stuff or advertise for personal benefit. While "swaps" where money changes hands aren't quite money making ventures, they're similar to things that have been shut down here in the past like people selling copies of WL lobby music for just a few dollars to cover the cost of the CD and mailing.
2) Any activity on such boards will be only tolerated until the "hassle" factor for moderators and the site owner reaches a certain threshold. Once that point is crossed, the tendency is for the mods to say "enough". Legalities and ethical questions about the now banished "swaps" aside, it was pretty clear that more and more people (even though a minority) were getting burned and losing money. Even though the DIS can post it up front that they are not responsible for member-to-member transactions made via the site, that won't stop people that feel they've been ripped-off from PMing the mods to seek information in an effort to try and track down their lost money or CD. It takes time and energy to deal with those situations. I also doubt that that mods and webmasters like knowing that more and more people were using the DIS as a tool for ripping people off. I think a threshold was crossed on several fronts, and the decision was made to "stop the hassle".
I also feel safe in saying that if the DVC rental board started to go south in terms of rip-offs that its activities might be halted too... I don't think it's immune from such triggers.
Lastly, I doubt that it had anything to do with the "bandwidth" that such swap threads were taking up. For the DIS, I sure they were a drop in the bucket.