Why 2.4GHz is free?

Answer 1

Defined by the ITU (International Telecommunication Union) originally designated certain unlicensed radio spectrum (frequencies) to be allocated for Industrial, Scientific and Medical (aka ISM) applications.

As other communities became aware of the these "free" radio frequencies (RF), many of the economical frequencies have been adopted and standardized by other industries (e.g., cordless telephones, microwave ovens, wifi/wireless networks, Bluetooth, ZigBee, etc).

Specifically, the 2.4Ghz you are referring to is available to the "Americas" (North & South America, Canada, Greenland, some Pacific Islands, etc) part of the Globe. In the US, the FCC further controls the (unlicensed) spectrum and limits its applications to low power transmissions.

In the US the ISM frequencies are defined as 900Mhz (915Mhz), 2.45Ghz, 5.8Ghz, & 24.125Ghz.

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