Many of those "sterile international/international transfer zones" that other countries have, require transit visas from citizens of non-West Europe/US/Canada - for them, it is just a regular part of making travel plans.

I suspect there aren't enough passengers who wish to avoid staying a few days in US/Canada in between their trip for many airports to have a such a secure area - unlike European or Pacific rim airports, which get many trans-continental passengers.

On another note - yes, it adds another $100 to your trip budget, but its extra security. If the OP wants to do a RTW, why not stay in the US a couple of days in between? - if you are getting the visa anyway.

Edited for typo.