Primarily they are immune because they are designed to be a much tougher enemy than the regular combat bots. - Also, during testing it was found that by making Spidertrons immune to saw blades, performance was able to be kept in check in the later tiers of Endless Mode. - Hopefully Spidertrons will be a bit less "flighty" with the upcoming hotfix, thereby getting stuck less often in those very difficult positions; e.g. between the walls in the saw blade trap.
-Cheers