What's the big deal with cordless phone frequencies?

Governments care about the frequencies that wireless stuff broadcasts at because there are only so many wireless frequencies to broadcast at. If the Aruban government has its military radios broadcasting at 900 Mhz, they wouldn’t want your cordless phone interfering with that.

If anyone could transmit at any frequency they wanted, no radio operated stuff would work, as everything would be interefering with everything else.

That’s why different devices or services have different dedicated frequency bands, for example
Baby monitor 49–50 MHz
FM Radio 98–105 MHz
Wifi 2.4–2.45 GHz

Cordless phones typically transmit/recieve on
46–47 MHz and 49 MHz (gen 1)
902 MHz and 903 MHz (gen 2)
2.4–2.5 GHz (generation 3, shared with wireless router frequencies)
2.4+5.8 Ghz or 900 MHz+5.8GHz (generation 4).

Typically one frequency is used handset->base station and another for base-station->handset.

For gen 4 phones the 900 MHz frequency is used for handset->base station transmission, and the reason for this is that it requires less energy to transmit at 900 MHz, thus the handset battery will last longer.

As for frequency allocation, as @Dominic says, each country has it’s own governmental body regulating the use of radio frequencies, in the US the FCC regulates the use of frequencies, i guess Aruba as a Dutch colony falls under different regulations. I don’t know what the use of the 900 MHz frequencies are under Dutch regulations, but I know that the 900 MHz band is in use for RFID chips for warehouses in many places in the world.