This is because your GRID value is set to 1 bar. If you set it to half notes, then you can select half a bar. For general operation, I would set the GRID to quarter notes(but that still allows you to select half a bar, by selecting 2 quarter note divisions). I suggest going to youtube and watching some videos on using the GRID in Pro Tools(seeing it in action is a much better teacher than me trying to explain it)
You may find that recording full bars makes more sense from a musical standpoint, but remember that the GRID value can be changed any time(so you can SELECT half of a BAR, even though you recorded a full BAR, or several BARS). As an example, if I want to use a short loop of drums, I might record 10 or 12 bars, then go pick the best 1 or 2 bars to use as a loop.