We all seen the Oracle vs Google lawsuit... sure they can choose to stand up to frivolous DMCA takedowns, but it doesn't means there won't be a frivolous lawsuit that follow.
It just not worth it, why takes that risk when taking something down protect you from it?
I mean, that's clearly the stance that most platforms are taking.
The "why take that risk" would be -- to stand up for your user's rights or interests, against takedown requests that are frivolous or invalid. Github actually has occasionally taken that stance.
@teraflop's point is that a company can choose to take that risk on a case-by-case basis -- say in cases you think are especially frivolous and thus also especially unlikely to be a legal liability -- and the risk borne is only with regard to that case, it does not imperil the ability to use the DMCA safe harbor defense in other cases.
(Github seems to be unusual in having already decided to take that risk a couple times on a case by case basis, I think? They can be commended for it, and encouraged to do it more, or have transparent standards for when they will do it, or whatever).
(Note that in the USA someone can file a frivolous lawsuit against you anytime. In this case, even if you respond to all takedown requests there's certainly no guarantee someone can't file a frivolous lawsuit against you anyway. It just shouldn't get very far, and will get thrown out quickly, if things work right, which certainly sometimes they don't, unjust things happen).
https://en.wikipedia.org/wiki/Viacom_International_Inc._v._Y....
https://en.wikipedia.org/wiki/Google_LLC_v._Oracle_America,_....