I think I understand the rationale behind seeing BDSM as an orientation, rather than a fetish. I think it's misguided to include it in the LGB grouping, though.
Potential reasoning is: person is "BDSM" (rather than lesbian/gay/bi/straight) in that they are only able to enjoy sex/be aroused if there's a BDSM element, with the sex of their partner/s being irrelevant. So the orientation is "BDSM", rather than lesbian/gay/bi/straight.
However, afaik, LGB groups exist to counter discrimination and, unless you want to tell people about exactly what gets you off (that's still considered poor manners, right?), being in a BDSM relationship is unlikely to cause you any problems - unless it's a same-sex relationship, in which case the discrimonation you're likely to face is due to homophobia, I would have thought? So the inclusion wouldn't really do anyone any good, but would distract from genuine social problems.
I agree that it's all a bit special snowflakey: "I might appear to be straight, white and middle-class, but I'm part of an oppressed group too! Let me join your club!"