Just to name a few micro with ISP features found on digikey.
Just click on them and go straight to the digikey page.
Why do you found only AVR as the only microcontroller with ISP feature ?
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There is no special meaning of "In System Programming" of Atmega series.
Other microcontroller mentioned above provides in system programming (without removing the chip from your board in order to be programmed in external programmer) as well.