10DowningSt
One reason for asking the CC is a quaint old-fashioned thing called courtesy.
Sadly, most CC don't share the depth of knowledge about GPS that you do. Given that both technology, and the specifications of models within established product ranges change all the time, it is unreasonable to expect CC to be au fait with each individual product. You may be an expert, or you may just have read the manual during taxi-out: CC have no method for assessing your individual level of expertise or the specification of the product. They do, however, have a reasonably good working knowledge of the various regulations and policies that are in force at the time. Those regulations and policies are in place to attempt to ensure a safe journey for yourself and your fellow passengers - they have been created by people whose depth of knowledge of aviation safety can be publicly demonstrated. How dare you sneer at those people, such as Cabin Crew who work to keep you safe.
Speaking of expertise - may we all know yours. You have stated that a GPS poses "no more risk to aircraft safety than a Biro pen, and perhaps less than an Ipod". Is this an approximation of the facts, totally accurate or an educated guess? I'd suggest that the latter is likely to be the case, but I stand to be corrected. Given your attitude to CC, it would seem likely that you're not actually a pilot either - meaning that your statement about asking permission is nothing more than a petty rant from somebody who simply resents any kind of imposition being placed on them by anybody or anything else.
Parents of 2 year olds will be familiar with this level of behavior.