Hi, and welcome!
So I stopped by the Little Free Library in San Diego and noticed that several books only had "BCID" and the number written inside in pencil. I understood it but I was looking; people who aren't familiar with BC probably won't get it. Is it considered bad form to pick these up and label them for the "owner"?
I have noticed that some BCers don't seem to take a lot of trouble to label their books - but I've usually seen this with books brought to BC meetups, where the releasers are sure that the books will be taken by other BCers who'll know what to do. But I still think they should do a better job of labeling the books from the beginning, as we never know where they'll wind up, and as you point out, if a finder isn't familiar with BookCrossing, simply seeing "BCID: xxx-xxxxxxxx" won't necessarily draw them in.
I don't think it's bad form to add labeling to such books; sometimes I've done that myself with those meetup-releases. (If you wanted, you could send a Private Message to whoever released the books to ask them if they minded, though it's a fine line between offering to help and sounding critical of their chosen labeling-style...)
I'm sure that some/many BC books aren't heard from again due to some combination of skimpy or hard-to-read labeling, loosely-attached labels that fall out, or hastily-written BCIDs that have transposed digits or can't be easily made out. But even with flawless labeling, some folks just aren't interested in logging the books online, so while good labeling helps in getting JEs, it's not a guarantee.