I am not sure why the term has been broadened to include such a wide array of things, but it has. I think, it may have to do with the movement underway that is trying to make BDSM more palatable to the general public. By grouping everyone involved in BDSM in any manner, together under a single word, it gives the community a "oneness" that is not always there. A commonality, that can be wonderfully useful, but can also be misused. 

The term now covers such areas as homosexual leather men and women, master/slave relationships, long distance relationships, online relationships, 24/7 d/s relationships, play partner relationships, and mentoring relationship. Basically, any relationship that has any amount of BDSM in it, no matter how infrequently, is a part of "the" BDSM lifestyle. 

Not everyone defines the term in this context. Some think it is too confusing since the term "lifestyle" can also be applied to a specific form of the lifestyle such as a 24/7 bdsm relationship. Some do not consider those who do not live a 24/7 relationship as a part of the bdsm lifestyle. How you choose to define the word is up to you.