All splitters are bi-directional. Directional couplers (TAPS) are a different story.
I guess that using the word "bi-directional" is supposed to make it sound more impressive or something. If you think about it, most people know that a bi-directional amp is needed for internet, VOD, etc. If they saw 2 different splitters and one was labeled "splitter" while the other one was labeled "bi-directional splitter," the one labeled "bi-directional" would probably sell quicker, even if it was a couple bucks more. It's all about the advertising.
The one that drives me more nuts is the term "amplified splitter." There's no such thing as an amplified splitter. A splitter is a passive device, no ifs, ands, or buts. No way of changing that. "Amplified splitter" refers to a standard amplifier with a splitter on the output. But, someone came up with that term and now it sells products.