Doubt in Amplitude Modulation
It doesn't matter what equation you use to describe AM, the actual physical modulation itself has an unsuppressed carrier, and a certain amount of power. It's a very simple modulation, one of the earliest, and as such is inefficient, but easy to use.

If you want a suppressed carrier, that's a different type of modulation, which needs a more complicated demodulator to read it.

You could make \$k_a\$ vary in the range 0 to 1, if you also modified \$A_m\$, and interpretted \$k_a\$ as something other than the modulation that you put on the carrier with a conventional modulator, and recovered from the carrier with a conventional detector. 

So you see it's easier to interpret \$k_a\$ conventionally, because then it maps onto what's typically happening as we use the system.