The z-index's are set to 100 because it makes the widgets compatible with more sites. The reason is that, if the z-index isn't set, on many sites, the Like button (especially the popup) will be hidden behind the content itself.
We've done some investigation and in the next release, we'll be changing the z-index to a default of 99. That's because we've noticed that many drop-down menus use a default z-index of 100, which causes unknown results as to whether the Like button will be above or behind it. With a z-index of 99, we think that will provide even better compatibility to be above the article content but below other common 'hovering' graphics. There will still be some sites with issues, and in those cases, we'll be here to help
Hope that explains!
Alex