It's always the diver's final decision. However, if dive shops are organizing a trip, they should make it very clear what the dangers are. They also shouldn't be trying to put "peer pressure" on someone to try and get their money. For example, downplaying the risks of a potentially dangerous dive to get someone go would be irresponsible.
A good (positive) example was one of my last dives, it was on a wreck. Before going down the Captain just said "if you don't have training in penetrating wrecks, I highly recommend you stay on the outside or just enter the small wheelhouse area (it was impossible to get lost in that).". No enforcement, but a clear directive about what was and wasn't a good idea based on training. Before I even got on the boat, I was discussing with the shop which dives I should go on, and the owner was clear which ones were dangerous, what the dangers were, and if they were likely beyond my ability. This is what I expect from shops, Captains, divemasters, etc: be clear and honest about the risks. Don't say "ah going past 100 feet is no big deal" or something to someone who hasn't been told anything about deep diving other than "you shouldn't do it".
I guess my point is you always see comments "ask a local ..." on here, so obviously there is supposed to be some element of trust, and with that comes some responsibility for the shop or whoever. No idea if that was the case here or not, but if we are expected to have trust in other divers, they need to bare some of the responsibility if you are unknowingly put in a dangerous situation. If we're not allowed to trust anyone, then sure the responsibility is all on you, but I'd think diving would be a lot less fun.
Whether they should strictly enforce certification limits is a different question. If there is a dive to 100 feet and the shop is quite clear about this fact (and doesn't downplay the dangers or give their blessing) and an OW diver wants to go on this, I now have to shift the burden to the diver. True, maybe the shop would be more responsible if they enforced the limits, but at least in this case the diver is made clear of what the dangers are.
Shop/charter honesty and knowledge + diver responsibility is the best equation for a safe outcome.