They might fall under Title I (which is the transcription of the WIPO treaty to US law), but most mentions of DMCA are about Title II (the "Online Copyright Infringement Liability Limitation Act") which only applies to service providers, not first-party content hosting.
First-party hosting still needs an ISP though, and that ISP can be sent DMCA notices.
What is the responsibility of the ISP to its subscribers in that case? I.e. would I reasonably be able to take legal action against them if they deny me the service for what turns out to be a frivolous DMCA claim?
> would I reasonably be able to take legal action against them if they deny me the service for what turns out to be a frivolous DMCA claim?
The point of the DMCA Safe Harbour provision is that the OSP is protected if they don't involve themselves and just proxy claims assuming their are in good faith/correct. So no. You may have a claim if the OSP does not restore service after a counter-notice with no followup legal action from the claimant but they're allowed 10-14 business days for that one so you may be out for 3 weeks without any recourse, but OSPs can also rely on their TOS to make that moot and even ignore the counter-notice altogether.
You probably won't even have a claim against the originally alleged copyright owner: the notice of copyright infringement only requires "good faith belief" under no penalty of perjury.
> You probably won't even have a claim against the originally alleged copyright owner: the notice of copyright infringement only requires "good faith belief" under no penalty of perjury.
That you don't have a specific DMCA-derived or declaration-under-penalty-of-perjury-derived cause of action doesn't mean you don't have a standard defamation-based cause of action.
> What is the responsibility of the ISP to its subscribers in that case?
Largely, to fulfill whatever is in their terms of service, though so long as they follow the counternotice procedures in the DMCA -- which require you first to file a counternotice -- they have no liability to you for removing content in response to a facially valid (even if substantively completely false) DMCA claim, even if they would otherwise under general principles of law or your terms of service (that's why the DMCA notice/counternotice provisions are called "Safe Harbor" provisions -- they provide a shield from existing copyright liability and liability to the user whose hosted content is removed, provided, in the first case, that the notice provisions are adhered to, and, in the second, that the content was removed in response to a DMCA notice and the counternotice provisions are adhered to.)
First-party hosting still needs an ISP though, and that ISP can be sent DMCA notices.