[Odsh]

Control Device is a legal matrix action that requires User or Admin level, depending on what you intend to do with it.

If it is a legal action, why is it resisted by Willpower + Firewall? Why would a device with better matrix defenses be more difficult to control?

When you use Control Device, you may take any action that would normally require the level of access you currently have.

That... doesn't make much sense, either you have an access level, or you don't.
I assume a difference is made between a legitimate User or Admin access level and one that has been obtained illegally. And I suspect the above test is meant for devices that the hacker gained illegally access to. Control Device being a "legal" action just means that it doesn't increase OS.

Is this the correct interpretation?

If yes, I assume that you need no such test to control a device that you own?

The Backdoor Entry action states that "you gain Admin access to the target, and it does not count as illegal Admin access". So if I gained Admin access to a device that I do not own with Backdoor Entry, does it mean that I can control it as if I owned it, i.e. without the above test?

[Xenon]

The way I understand it, a difference is made between a legitimate User or Admin access level and one that has been obtained illegally. And I suspect the above test is meant for devices that the hacker gained illegally access to. Control Device being a "legal" action just means that it doesn't increase OS.

Correct.

If yes, I assume that you need no such test to control a device that you own?

Correct.

The Backdoor Entry action states that "you gain Admin access to the target, and it does not count as illegal Admin access". So if I gained Admin access to a device that I do not own with Backdoor Entry, does it mean that I can control it as if I owned it, i.e. without the above test?

It means that if you probe a host and then sneak in via a back door (instead of brute forcing and rushing through the main gate) you will not automatically generate OS every turn. As a hacker you still need a Control Device action to take control of a device that is linked to the network.

[Odsh]

Thanks for the quick answer.

So instead of outsider/user/admin, we in fact have outsider, legit user/admin and hacked user/admin. I hope this distinction is really worth the confusion it generates.

[Stainless Steel Devil Rat]

There are no rules for assuming legit ownership of a device/host.  There was a rule in 5e for devices, but you could never assume ownership of a host even then.

Admin accessto doesn't make you the owner.  It doesn't do anything more than unlocking certain matrix actions.  Admin access doesn't let you add/remove employee users to the host, call off the IC, and so on.  Hacked admin access is always hacked, and by the nature of the matrix it only lasts a short amount of time.

[Odsh]

I'm ok with the transient nature of a hacked Admin access level and the difference with true Ownership, if only for game balance.
The part that is a bit weirder i.m.o. is gaining through hacking a User/Admin access level, but not being able to do freely the legal actions that the User/Admin access level allows you to do. I guess that's just another example where I shouldn't project my real life knowledge and experience with computers to the matrix rules. It probably wasn't as confusing to me in previous editions where access levels were replaced by "marks".
It becomes quite obvious by reading further through the matrix chapter though as there are many other actions like that. Strangely, some actions do not follow this rule, like the Change Icon action.

That being said, it's also strange to have a device resisting a matrix action with its Firewall, but GOD being unaware or not giving a damn...

EDIT: everything would have been much clearer had they renamed "legal" to "covert" or "traceless" i.m.o.