You have probably noticed that Avidemux has several "modes" of work. Cutting/splitting videos without re-encoding can only be done in "Copy" mode.
It also means that no video filters can be applied.
The "Copy" mode works by copying the raw compressed video from the original file (container) into the new file (container). That's why you can't use video filters in this mode.
You might have noticed that this cutting doesn't actually work always with exact precision. Effectively, "Copy" mode can only split on keyframes, and that means that unless you chose a keyframe to split on, your resulting video will have a few more frames than what you intended.
Usually too few to notice, but they are there anyway.
However, with some specific codecs, Avidemux can do "Smartcopy":
If your file uses MSMPEG-4 (DivX ;-) 3) or MPEG-4 video, you can use smart copy mode. Smart copy will leave most frames unchanged, but re-encode the part that lost its reference images with a fixed quantizer. So in our example, frames ab and nop would remain unchanged, while frames jkl would be re-encoded to use a different reference frame. Smart copy will ask you for a value for the quantizer to use – 4 or 5 is generally OK.
So you still get some video re-encoding, though minimal.
PiTiVi is a Non-Linear video editor first and foremost, and so it doesn't have a "copy" mode, which would prevent any track-mixing and any video effects. Which I think is good, because people would turn it on, and you would get complaints that the video effects don't work :)