A slippery slope argument states that a relatively small
first step leads to a chain of related events culminating
in some significant effect.
6 strikes is neither a relatively small first step, nor is prosecuting people under it a grand leap (it's the next logical step). Slippery slope isn't a tool for dismissing any claims of cause and future effect, it is an informal rule for when claims of cause and future effect are permitted, namely when one can "demonstrate a process which leads to the significant effect". In this case, "language of the agreement between ISPs and the entertainment industry" (http://www.dslreports.com/shownews/Six-Strikes-Official-Says... and http://www.dslreports.com/shownews/Will-ISP-Six-Strikes-Incl...) clearly states that ISPs have the full authority to report 6 strikes offenders to copyright owners, should they choose to prosecute them:
The Participating ISP will, however, continue to track and
report the number of ISP Notices the Participating ISP
receives for that Subscriber’s account, so that information
is available to a Content Owner Representative if it elects
to initiate a copyright infringement action against that
Subscriber.
No demonstration of said process is necessary because the right by IPSs and copyright holders to engage in this very process has been clearly reserved. Not a slippery slope.
If you come into a debate that's been running for years and observe one extrapolation by one commenter, sure that may appear to be a slippery slope fallacy. In reality, the slippery slope is being actively implemented by the various media companies in a decades-long campaign of copyright maximalism. The periodic copyright term extensions, the DMCA, ACTA, TPP, lawsuits against end users, etc. are evidence of an ongoing effort to shift public perception and behavior.
So no, it's not a fallacy in this case to expect the MPAA/RIAA to keep alternating between mild and extreme tactics, just as they have been doing.
