Presumably there's no legal reason why the ISPs couldn't write to all their customers giving "notice of upcoming partial internet service outage, due to the actions of La Liga". It would be factually true
Of course, LL could still give them hell in court even on false grounds (and maybe even win anyway, given the case detailed in the root comment). And in any case there's simply no commercial reason why they would stick their neck out in the first place
I think most of this are being done in the moment, without advanced warning. Plus, some ISPs carry soccer in their TV offerings so they’re probably not benefiting from speaking out. At least, my ISP does replace the blocked website with a notice explicitly stating that this is the result of a judicial ruling in favor of LaLiga
Of course, LL could still give them hell in court even on false grounds (and maybe even win anyway, given the case detailed in the root comment). And in any case there's simply no commercial reason why they would stick their neck out in the first place