Dude, just use common freaking sense here.
Wild animals are going to eat whats available- what do slow moving torts, strictly "herbivorous", eat when the droughts come and all vegetation dies? Do they migrate a few hundred miles to greener pastures? No, they make use of what theyve already eatten and go looking for alternate food sources.
They do what they must to survive. Basically, instead of thinking of your tort as a classic "tortoise", think of it as what it is: a big-ass land turtle.
And what do turtles eat?
Well, if youve dealt with turtles, you know the answer to that is another question: What WONT turtles eat? Christ, Ive seen turtles eat everything from frogs to strawberries, earthworms to guppies, rose pettles to pumpkin flesh- basically, they eat whats they can get.
Now, granted, torts are rather different. But still, wouldnt you think that they would be at least SOMEWHAT omnivorous? I mean, hell, if you can show me an animal that feeds on only one type of food, Ill be surprised.
Pretty much all animals are omnivorous- they eat what they get. As the fellow before said, if you offered your tort commercial dog food or monkey chow everyday, yeah, youd prolly mess him up quite nicely.
But how can you not see that in small doses it is beneficial? Most animals dont get ALL their nutritional needs from just one source, because theres very few creatures on the planet that serve as a good long term food source by themselves.
Savannah monitors eat dandelions and tegus eat raw onions.
Cave salamanders in the Ozarks have been found to consume bat feces as a means of staying alive- was that their original food source?
No, they learned that it had something beneficial to them, or they wouldnt eat it. Is it a good overall food source?
No, thats why the salamanders back it up with small inverts/anthropods.
Hell, grazing herbivores like cows and bison HAVE been known to consume dead herdmates. Not to mention, reality check here:
You ever mowed your lawn with a bag mower? Ever seen ALL the bugs that leap out of the way of the mower as it comes towards them?
You ever looked in the bag? If so, you would see thousands of little grass dwelling insects, and there would be far more that you wouldnt see.
Now imagine a herd of bison or cows grazing... dosnt it seem obvious that they would consume quite a bit of insect matter over time? prolly never enough in a single sitting to really constitute a large portion of their diet, yet still a beneficial addition to what they normally consume.