You're absolutely right. I did the following just prior to opening the forum and seeing your response. Here's the story:
I woke up this morning realizing that there's no good reason for Synaptic to grief while adding bchemnet as a repository. After all, this the second installation of the same version of linux Mint on the machine, and on the first installation I had no problems with this process. So, I must be doing something incorrectly this time round.
Fueled by some hot french roast and a danish, I revisited this issue by trying to add bchemnet as a repository via the gui and the command line, monitoring sources.list as I went along. I saw that adding bchemnet via the command line updated sources.list without error, yet trying to add bchemnet via Synaptic caused grief. So it must be something I'm doing incorrectly in Synaptic.
Then, poking around in the Symantic Software sources dialog box revealed that bchemnet is in fact an added and enabled repository. However, it's listed in the "Additional Repositories" section, not the "PPA" section. So this problem was caused by operator error: I should have been adding the bchemnet repository as an "Additional Repository", not a "PPA". I verified this by removing bchemnet in Synaptic, confirming that sources.list contained no references to bchemnet, then added bchemnet using the "Additional Repository" section. Synaptic added and enabled bchemnet without grief.
I don't know if it may add value for someone to edit the "Setting Up the Repository" section of
http://www.bchemnet.com/suldr/index.html such that it mentions adding bchemnet as an "Additional Repository" instead of as a "PPA". Then again, that's what this forum is for.
I hope this entry at least provides some small measure of entertainment at my expense... I continue to live, learn, and take better notes. Zim is helpful in this respect. Cheers.