This same bootless argument goes on in my other hobby field as well. I build violins, mandolins, and guitars from "scratch", meaning that I start from raw lumber flitches and carve everything pretty much by hand. Over the past several years, kits have started to become available that have the basic components roughed to shape. They still require substantial work to yield a decent playable instrument, but those that work from kits are looked upon by some as mere "assemblers". Purists would look down on me because I purchase flitches already cut and aged, rather than cutting my own maple and spruce logs, and I don't mix my own varnishes. To me, it's all a matter of degree. In the larger world outside RC, it's generally accepted that things produced from plans are "built" and prefabricated kits are "assembled". After all, an ARF is nothing more than a kit that someone else has partially or mostly assembled for you. Very few "builders" build their own engines, wind their own motors, build their own radios, or machine their own retracts, so who gets to draw the line where building becomes assembling? Would you say a plastic model airplane "kit" is built or assembled?
Who cares? It's like arguing how many angels can dance on a pinhead. A guy who flies his ARF has just as much fun as the guy who started from a kit, or one who designed and built from scratch. You might legitimately feel more PRIDE in the extra work and skill you put into your scratchbuild, and I think everyone recognizes that much more skill and industry is required for kits or scratchbuilds, but that's an issue separate from the semantics.
That's my $0.02 as a scratch, kit, and ARF builder/assembler, and it's no more valid than any other.
Now, the toy thing... them's fightin' words boys.