I believe it has to do with international copyright laws. For an in depth explanation, see here:
http://en.wikipedia.org/wiki/DVD_region_code
There are region encodings and then there are video formats (NTSC and PAL). Most of Europe is PAL, whereas the US is NTSC. So the region free players will let you play both PAL and NTSC formats. 99% of all "normal" store bought DVD players will not have the hardware do decode and convert a PAL signal to NTSC.
And yes, the Tick animated is one of my all time favorite cartoons. If you liked the Tick, you will likely love the Venture Bros. :yes:
Bart