For starters: because they are in fact not assumptions, as has been posted many times by various people, myself included.And your assumptions are correct because?
The taco is mounted inboard of the chainring which actually causes it to come into contact with rocks less often than the outside the chainring mounted bashguard. it is of a lower profile (smaller effective radious) and therefore also avoids contact. bashguards are mounted to crank spiders which are not all designed to withstand the kind of forces placed on them when a very sturdy basguard hits a rock. many of them bend very easily, especially the lighter DH models everyone seems to want these days. From personal experience my gravity lite spider bent several times before i switched to the lg-1 and taco. Mounted to the iscg tabs on my Sunday (contrary to your assumption, they are extrememly sturdy) and smashed into rocks and logs regularily since last May the taco has niether come out of place nor bent or damaged the mounts.
Also, mounting to iscg tabs allows the bolts to be tightened just enough so the guide/taco can rotate a bit if you nail something hard enough (same reason you don't clamp your brake levers down full force), the standard bashguard cannot rotate and transfers all the force, with more leverage, into your crank spider which is usually less sturdy than the iscg tabs you seem so worried about.
Now, if the frame manufacturer is too incompetent or lazy to make sturdy tabs then that's another story, but it doesn't take a whole lot of work, material, or weight, to make high quality mounts; and even less sturdy mounts can stand up to a considerable amount of abuse if you set things up correctly.
So, to reiterate: This is not an assumption.