Maybe if you started off without internet at all and then got broadband--instead of upgrading from something else to broadband--it's very likely you either have no antivirus or out of date antivirus (after all, if you had never been online or connected to other computers, you'd have little need for it).  Even if this doesn't apply to you, the fact that it applies to some people would mean they would need to tell you this precaution just for being a customer.  A virus is the only thing I could think of that the internet would do to harm your files.

That's just a guess, but it makes sense to me.

That really works well with my idea though:  if you're getting internet for the first time and you have NO IDEA WHAT YOU'RE DOING (and think you don't need an antivirus), you're likely to have everything ruined when you start thinking people are saving you money on replica watches.

If the ISP says to backup your files, it probably helps them to not be legally responsible for you being a dipshit (not that they should be in the first place).

No, it would just be for a worst case scenario. A recommendation is all it is.