A drabble is 100 words. It just is. If it's 300 words long, or 500 words long, or 98 words long, the term you're looking for is "short story" or perhaps "ficlet" or "flashfic" but it is not "drabble." It's exactly like saying, "Well, I like the sound of the word 'sonnet' and it sort of seems like it should mean 'Any poem in iambic pentameter that rhymes,' so I'm going to use it that way."
People who say "Er, no, that's not a sonnet; it's a very nice poem, but it fails as a sonnet because it has too many lines," aren't being tightasses, they're just making a statement that what's been written here doesn't fall under the rules of that specific literary form. Nobody's saying it has to comply with a specific literary form, but if you don't want to be limited to the rules that have been established for that form, all you have to do is not call it that (or in the linked case, admit that it was an honest screwup). It's a problem that solves itself.
I think what bugs me about the way 'drabble' is being used recently in fandom to mean 'any really short story' or at best 'any short story of a pre-assigned round number of words' is that when done well, the 100-word drabble is a lot like a very carefully-crafted poem. I haven't regularly written poetry for years, but it was my first love and it's still the art-form I have the most formal study in, so I love drabbles that recapture that feeling (whether I wrote them or am reading them) of crafting and shaping where every. single. word. and its placement counts at a very intense level. Use of 'drabble' to mean any old short fic bothers me because I sense that the meaning-shift has come because the word sounds like "some little thing that dribbled out of my pen," and that association then attaches itself to the 100-word ones as well, that people have worked hard on to make meet the requirements of the form.
(And I'm not saying people didn't work hard on the 120-word stories and the 500-word stories; I'm just saying that they didn't specifically work hard to make a story that says something touching or clever and says it in exactly 100 words, no more, no less.)
Edit: But then there's people who don't know the original meaning of 'drabble' because they've only seen it used in fandom, by someone else who learned it from someone else who... etc. I'm not bitching at them (or even really at the people who know the literary meaning and have tried to adapt it to different word counts) -- I'm just sort of bitching into the wind that this is what happens when word-meanings become more and more inclusive, moving from the specific to the general. We lose, as julia_here pointed out, meaning, in our quest for inclusiveness.
I like words.
My cat's breath smells like catfood.
- A dribble, a drabble -- below, please find babble...