Everything in business can be explained with benefits. They don't see the benefits. Which makes sense for everything you said. Unless the location is a strong selling point, why worry about it. It's just as likely to sway a person away from a pipe as it is towards one. Especially someone who doesn't know much about pipes. A lot of people are surprised that Italy has a pipe culture, everyone knows England has a pipe culture. Yet if you're dead set on an Italian pipe, you won't have any trouble finding one.I don't think that lacking a stamp means you can necessarily conclude that they're trying to hide something.
Other possibilities:
• Stamping a pipe takes tooling. Tooling needs refreshing. One less stamp to have to keep up with is one place you have to spend less money in production—financial.
• There's lots of info stamped already, for many this is a logo and a shape number or model name. Adding a "made in _____" increases the total area taken up by stamping, possibly making the stamping look too busy—aesthetic.
That's just off the top of my head, there's a plethora of other possibilities like, maybe they don't feel like they need to stamp because most people know? Maybe they just didn't think about it at all because they're not patriotically minded?
Just like if a business runs a sale, that doesn't necessarily mean they're trying to clear inventory or move something that isn't moving or trying to motivate people to buy a crap product.
Not everything is insidious or ill-intent... and this is coming from a professed skeptic and misanthrope.
Don't forget that pipes are international too. They have to consider many markets.