How does a single-SIM MVNO on mulitiple networks work?

So if you get straight talk, you have to select from 4 SIM cards to pop in, and that activates your phone on that particular network... I get that. Lately, however, I've seen multi-network MVNOs arise (like Defense Mobile, ROK Mobile) that only use one SIM card. How does a single sim card authenticate between several carriers? How does a single sim multi-network MVNO handle that extra step Verizon usually requires of authenticating the individual phone's IMEI with it?

Please help me understand.
Thank you.

How does a single-SIM MVNO on mulitiple networks work?

So if you get straight talk, you have to select from 4 SIM cards to pop in, and that activates your phone on that particular network... I get that. Lately, however, I've seen multi-network MVNOs arise (like Defense Mobile, ROK Mobile) that only use one SIM card. How does a single sim card authenticate between several carriers? How does a single sim multi-network MVNO handle that extra step Verizon usually requires of authenticating the individual phone's IMEI with it?

Please help me understand.
Thank you.

Two identical WiFi notifications for click-through-UA free WiFI networks

I have Startbucks WiFi added to the list of WiFi networks on my S2. As many of you probably know, Startbucks WiFI requires users to click through an "Accept and Connect" screen before allowing them to access Internet. Now, I notice that when I walk into a Startbucks with my S2, it pops up two identical standard notifications about being unable to access Internet and WiFi network requiring user attention (or something along those lines).

So, why am I seeing two identical notifications every time? Is this something about my setup? Or is it just a quirk of Samsung version of the ROM?