Bryant has pretty well covered your questions. I can add a few things.
1. Does anyone follow code? Do you have pictures of any of these builds?
2. How can you tell if a main floor with a basement can take the weight? Don't really want to do trial and error here ( wait and see if the floor sags... yeah right)
3. How come all these builds are right next to walls, how does that work without creating "FIRE, FIRE, FIRE" as mentioned in the book, or all the stoves so new that we don't know yet?
The only code that sort of addresses RMH construction is the International Building Code section R1002 Masonry Heaters:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_10_sec002.htm
However, there a few aspects of RMH construction that cannot follow code, especially the door requirement.
ASTM E1602, which is incorporated by reference into R1002, can only be bought from ASTM for $44:
http://www.astm.org/Standards/E1602.htm
I have found a copy online, but it appears to be pirated.
It does describe open-firebox clearance requirements, so could be useful in negotiating with inspectors if necessary.
The masonry heater code only mentions flue-channel types and not bells which are becoming a common RMH style, but this should be a minor concern in relative terms.
The Portland RMH code has apparently been adopted, but I have been unable to find a copy of it online. Ernie and Erica Wisner talk about it here:
http://www.ernieanderica.info/rocketmassheaterpermitting
A bench-style RMH may be able to be supported on a sound, well-built wood floor in certain conditions, particularly if the bench is spread across a large number of floor joists and is relatively near a supporting wall below. Code will probably require noncombustible support in any case, if you are following it. A bell-style RMH which is taller than wide will definitely need its own foundation.
RMHs with barrels near walls are generally in cob or masonry buildings. Some may be built by amateurs with unsafe clearances. The thermal mass portions should follow masonry heater code, except that there is in reality no need for 8" or more of masonry between heat-exchange flues and exterior surfaces in most places. The proposed RMH code that Bryant linked to gives appropriate clearances.