Because, last time I checked, your mom's PC didn't have a Bitcoin ASIC in it.
Same is true with GPUs. Never seen a botnet of 5870s. Whoever is geeky enough to buy a 5870 knows how to be secure usually.
Quit the marketing and BS talking. You are not using custom ASIC as that requires millions to produce and nobody would take that risk.
Probably using sASIC in the best case ...
Also, this rig box and box that makes 50 ghash/s or 20 ghash/s will only centralize BTC to the few that can afford paying over your threshold amount of 10k+ which I doubt many "casual" miners are willing to pay. Stop trying to sound like you are making BTC more secure because this simply is not it.
ASIC will be devastating to BTC and make it 51% easily in the early true custom ASIC times until everybody gets same HW and drops GPUs.
ArtForz mined about 60k BTC in the days everybody was CPU mining with a single 4870 AFAIK. Someone with secret ASIC could do the same and 51% this easily taking all our coins.