There's a LOT of problems with TracFone and AT&T right now. I've been keeping track of this ever since this crap started. There are rumors and people claiming things but it's nothing more than speculation.
Now, there are currently two issues with ST and AT&T:
1. ST has no AT&T sim cards.
2. ST is throttling AT&T users for no reason.
I'll address them separately with as much information as I can find.
Issue 1
This all started when the Nexus 4 went back in stock on Google Play. So many people jumped ship from other carriers to Straight Talk that ST ran out of AT&T cards. No one pulled them, no one told them to stop selling, they actually ran out of stock. The reason why they didn't run out of T-Mobile is because T-Mobile doesn't have as much coverage as AT&T does. T-Mobile simply doesn't work in as many places so AT&T was the best bet for quite a few people. However, they haven't replenished their supply for whatever reason.
Those reasons range from a possibility of a lot of things:
A. TracFone is slowly moving to T-Mobile and is using Straight Talk, one of their most popular GSM MVNOs, to start it on. Thus, they don't need the SIMs anymore.
B. AT&T is refusing to supply with more cards simply because they would rather not have more prepaid customers as they are losing customers to Straight Talk, Net10, and other MVNOs that simply use AT&T's towers.
C. AT&T is upset with the fact that ST is lying about having unlimited data when they should have a hard cap of 1.5GB - 2GB/mo, they have ETF amnesty, and Walmart (and ST) are selling the GSM unlocked iPhone 5 with 0% financing.
D. AT&T is running out of space for 3G and once they start getting more LTE, they can give cards to ST so that we can use the 3G.
Any one of these could be it.
AT&T owns a lot of stake in America Movil, the company that owns TracFone so they technically own ST as well as Walmart. So I don't see them trying to get rid of the MVNO.
However, this leads us to issue 2...
Issue 2
TracFone is not happy with AT&T since they throttled Net10 for no reason. They haven't given TracFone a reason, they just throttled them and that's that. This does NOT apply to Straight Talk. However, there are ST reps on Facebook confusing ST with Net10, claiming that ST has a cap of 1.5GB/mo. This is not true as it's still officially "unlimited" data. and it has not changed.
Though, Straight Talk has started to throttle users for no reason. Even if they've never done anything wrong. Some people who call ST or talk to them on Facebook are told that Straight Talk no longer supports AT&T. This is coming straight from the reps. No one is denying or confirming this so no one knows the truth there.
Some are claiming that Straight Talk is trying to force their AT&T users away to T-Mobile. It's actually been said that TracFone is tired of AT&T's crap and wants to move over to T-Mobile permanently for their BYOD plans. I'd say this is speculation, but at this point, it's looking like the truth.
People are even starting to sue ST for bait and switch. Their argument is that they advertise not only unlimited data BUT the same data you get on AT&T, Verizon, T-Mobile, and Sprint. However, with throttling, this makes your data "limited" via the throttle, making it completely unusable.
There are also rumors that it's AT&T controlling the throttling and ST no longer has control. That AT&T is doing this simply because they don't want anyone on prepaid anymore.
As for America Movil, the company that owns TracFone (and thus Straight Talk, Net10, and other TracFone based MVNOs), they are currently being investigated by the Mexican government due to the huge monopoly they have in Mexico. The shares of the company have started to fall and the company is starting to buy back shares.
America Movil 2010 Mexico Dominance Ruling Upheld by Agency - Bloomberg
Slim's America Movil slumps as Mexico advances with reform | Reuters
Mexico's Carlos Slim increases stake in America Movil: SEC filing | Reuters
America Movil (and ST by default) is in some major trouble, it seems.
All in all, this is one big cluster---- and I don't see it getting better. If you're throttled, here is a list of AT&T MVNOs to go to:
https://en.wikipedia.org/wiki/List_of_United_States_mobile_virtual_network_operators
Keep in mind, these towers use AT&T's prepaid while Straight Talk actually uses AT&T postpaid. Your service will go down considerably.