Well it is true. But why do adults need their own coloring books? Just pick up something from the nearest supermarket. But have you looked at the art supply section in your local store? Pretty much the only stuff you'll find are coloring books for children. Unless you are a huge fan of Disney, you will want coloring books made for adults!
I don't know how much of the 'coloring = therapy' is true but I do know why I love it. I loved drawing and coloring as a kid, even went to art classes for some time. But as often happens with life going by, I lost time for it in favor of other pursuits. Now I've re-discovered my love of art. Yes, I said art. To those who pooh pooh coloring and say it isn't 'art' by any means, just because I follow a recipe doesn't mean I'm not cooking.
Coloring lets me play with color without having to draw something myself. Not all of us can draw, paint or sculpt but should that stop us from enjoying the process of creating a piece of art? True my coloring may never grace the walls of the Louvre but I can still frame it and hang it up on mine! It has even sparked my interest in drawing again.
Coloring lets me relax and unwind. Many of us are familiar with coming back from a vacation only to realize we're not actually relaxed. That's because while your body may be lying on a beach, your mind may still be filled with all your worries. It's why meditation is good for you - it lets you clear your head. Coloring is meditation for me. It's mundane enough that I don't have to concentrate on each stroke but also involved enough that I don't think about anything else but the lines, color and my strokes.
I've been learning new techniques and experimenting with different color palettes. It's slow going with each image taking as many as 20 hours to complete! Adult coloring books are nothing like the ones we used as kids. Some of them are so intricately drawn that my color pencils can't possibly fit in the spaces. That's why I also use pens for the details, though pencils will always be my first love!