Merriam-Webster's New International Dictionary, Second Edition (M.W.N.I.D 2) with a copyright date of 1953 does have such a sense of the word, but it marks it as obsolete.
- (Obs.) Since; considering that; inasmuch as. B. While on the other hand.
However, what I think they meant is that it means whereas, which Oxford Dictionaries Online marks as a possible, informal interpretation of the word "where". Whereas would make considerably more sense as it can mean.
- The thing being so that; considering that things are so; implying an admission of facts, sometimes followed by a different statement, and sometimes by inferences or something consequent, as in the law style, where a preamble introduces a law. — The American Dictionary of the English Language (A.D.E.L.) by Noah Webster and published in 1828
In our context it introduces consideration of the subsequently stated fact. Also worth consideration is that "as" can also be a synonym for since, according to the Century Dictionary and Cyclopedia. A definiton of "where" as "whereas" is in the American Heritage Dictionary 5th edition too, yet not the fourth. It's in Webster's Revised Unabridged Dictionary 1913, yet not A.D.E.L., nor Merriam-Webster Online or the Merriam-Webster 10th Collegiate edition from 1993, although it can be found in the 1914 Century Dictionary and Cyclopedia. It is not in most other dictionaries I can examine online and I doubt it'll be in most of my other printed sources
In consideration of those facts and that M.W.N.I.D. 2 effectively marks this usage as obsolete, that lends me to believe its a resurgence of a fad from around the early 20th century. If you want my personal opinion, it is a fad that probably should have been forgotten, because it reduces clarity by far too much for a mere savings of two letters. I am guessing if it said whereas from the beginning, there would be little to no need for the question to be asked. We probably kept the two words separated more often for that reason.
The reason whereas might mean this probably has more to do with the "as" than the where. The Century Dictionary and Cyclopedia gives some further explanation in its suggested synonyms for Since:
Synonyms Because, Since, As, Inasmuch as, For. Because (originally by cause) is strong and the most direct. Since, starting from the idea of mere sequence in time, is naturally less emphatic as to causation: its clause more often precedes the main proposition. As is still weaker, and, like since, generally brings in the reason before the main proposition: as or since the mountain will not come to Mohammed, Mohammed must go to the mountain. Inasmuch as is the most formal and emphatic, being used only to mark the express reason or condition. For follows the main proposition, and generally introduces that which is really continuative of the main proposition and of equal or nearly equal importance, the idea of giving a reason being subordinate.
No mention of where in there or even whereas. I have not heard this used too often so I am not sure why the dictionaries are starting to include where as whereas in their pages again.