I reproduce this problem. In TCC, the shift-arrows select text on my command line as expected, but in the TC tabbed environment, shift-left and shift-right just act as the left and right arrows, moving the cursor without selecting anything. I'm running an intel core i7 (with only 0-5% CPU load while I tried the shift-arrows) so I don't think it's a matter of a slow CPU. Windows 7 x64. Is there any sort of debugging setting that I can turn on to provide you with more details?
By the way, I notice that shift-up, shift-down, shift-pgup, and shift-pgdown don't work to select rows or pages of text, even in TCC. Is this by design? (PowerCmd supports this, and I often use it in other apps, too, like MS Word).