This always gets me.
Of course if you don't want the DM to set-up your gear, all you have to do is politely tell them that you prefer to do it. It is a service and a courtesy that many dive shops will set your gear up for you. However, that does not in any way exempt you as the diver from the responsibility of checking your gear BEFORE you get in the water...including taking a few breaths off your regulator. It is YOUR responsibility to make sure your gear is set up properly and that your tank is on, etc. Think of it as an extra checks and balance system. Had this diver done this, the problem could have been easily resolved before getting in the water.
That is very odd that a DM would not remove the tape....and I don't see how the reg would even get a seal with the tape there...doesn't make sense to me. Anyway, an error like that is definitely the exception rather than the rule. Aside from that, setting up equipment is not rocket science and these DM's do it every single day...so it's not like they don't know what they're doing.
I always tell my divers that the crew will set up their gear unless they have objections to this...but it is still their responsibility to check it before getting in the water...or if they have a special configuration, set it up with the DM the first day, and then they can remember how the diver likes it for subsequent days. I even have this stated on the waiver.
IMO, if you don't trust the DM to set up your gear (which is a basic function), you probably shouldn't be diving with them to begin with.