What constitutes proper subject matter for postage stamps?

A couple weeks ago the Washington Post made note of some turmoil in the stamp collecting community regarding the selection of images to be used on forthcoming commemorative stamps in the United States:

As the U.S. Postal Service prepares to issue a stamp featuring Rudolph the Red-Nosed Reindeer next week, a postal expert whose 12-year term on the Citizens’ Stamp Advisory Committee ended earlier this year pleads with his former colleagues to resist the temptation to choose new stamp images “with the same profit motives as Big Macs, Slurpees, jeans or neighborhood tattoo parlors.”..

This airing of dirty laundry in the small but passionate stamp community... draws another fault line in an ongoing debate over whether the cash-poor Postal Service should pursue commercial stamp subjects to lure new collectors and revenue at the expense of more enduring cultural images...

The friction came to a head last fall, when the stamp panel grew concerned about how the Postal Service’s marketing staff was pushing pop culture that culminated with the release of stamps honoring Harry Potter...

“That said, while continuing to commemorate historic events and individuals, it is critically important that we offer subjects to interest younger generations and topical collectors into stamp collecting, such as Harry Potter, Janis Joplin, Jimi Hendrix and, most recently, Batman,” Saunder said.

Rudolph the Red-Nosed Reindeer debuts at the National Postal Museum on Nov. 6.

I remember the Harry Potter stamp points of argument, which included not only the commercialization of philately, but also the "promotion" and "glorification" of witchcraft.  Then there was the controversy six years ago when a politically-correct stamp design took the cigarette away from Bette Davis.

I find it interesting that the current controversy over U.S. stamp designs follows by several months the apparently not-very-controversial issuance in Finland of postage stamps commemorating the artwork of "Tom of Finland," whose subject matter [see top image] is of a genre that would set off a firestorm of complaint in this country.  The Finnish stamps are available for purchase in the U.S., but of course not valid for postage here.

Last year the postal service in Finland issued a set of four stamps picturing the "prettiest outhouses in Finland."