indoor carpets (i assume) would have to be made with consideration that people/pets(dogs/cats), and children will be on them on a regular basis. does this apply to reptiles, who knows, but keep it in mind that it is safe for just about everything else.
the only possible thing i could imagine is if you had a VERY hot basking spot and carpet slowly "burned" over time. Much like the fading i see in my cage materials from constantly being on the hot end of heat lamps. Things like this usually release unfriendly gases when melting or burning.
personally, why anyone would use substrate that has to be removed to be cleaned is beyond me. Use something that can be spot cleaned. it will save you so much time.
if u still insist on carpet, i would have to imagine ur snakes would be fine.