30+10=40. It's a 40 day window no matter how logical you want to get. But it's a strict 30 days. That's it. That's the way it is. It's not difficult for me at all to take the logical step that even if you call it 30+10 it's still a 40 day window.
When I booked on my 60+10 window, it was really 61 days from my first day in park. That's a bonus of being on site.
Onsite is 60+10 because you could be staying for 14 days, but you're still only allowed to book 10 days beyond. Same for 180+10 ADR for onsite guests. You can complain that Disney knows you're staying 14 nights, so it wouldn't be that difficult for them to open up 4 more days for you. But they don't. That's the way it is.
Your argument was 'How could they possibly know what your stay is .... 30+10 days from what !!??? I mean how is this multibillion dollar company supposed to know when you are going to be visiting WDW if you are staying off site ?? Garsh, its so confusing'
The simple solution, that you can't seem to make the leap to ... 30+10 starting the first day you book a Fastpass, I am guessing that's when you are visiting WDW if you are staying offsite, starting the first day you book an FP for up to the number of days you have tickets for (since Disney does have that data).
We aren't talking about "the way it is" the OP was questioning WHY its the way it is, and to an extent complaining that it is the way it is, and rightly so.
So onsite gets a 70 day booking window, offsite gets a 30.
It does not make sense.
It makes perfect sense to give onsite some advantage, or encourage onsite bookings I guess (though other people have argued quite well you are each paying the same for park admission regardless).
But, for a system that is designed to try and lock people in, and to let Disney know when people are going to be in the parks, it would make much more sense to let off site guests book 30+10
Let alone to simply the process for the guest, who under the current rules has to long on at midnight every night at 30 days out for as long as their trip is in order to ensure the best chance at FPs (though I don't know if this is really necessary to get the FPs they want)