A couple things. First is that for the drum magazine to hold more than 10 rounds you would need to be law enforcement and meet an exemption.
Second even if you were law enforcement the 'fixed magazine' of over 10 rounds would have made that an illegal assault weapon, and it would have to be used without a bullet button.
Law enforcement is exempt from the 10+ round magazine ban, but not exempt from assault weapon legislation, and a 10+ round magazine in a fixed magazine firearm is an assault weapon.
I could see some 10 round drums being created simply to retain that iconic look rather than actually being any more useful than a smaller box magazine.
The bullet button is activated by the use of some item small enough to insert into the hole in the middle. That is where the magazine release button is located. It is intentionally too small of an opening to be operated by a bare finger, thus requiring a tool, which was deemed by a ruling in CA years ago to not be considered a detachable magazine. Removal of the bullet button while it has any other prohibited feature such as a pistol grip turns it into an illegal assault weapon punishable by 10 years in prison.
It was created because there was a gray area that needed to be defined to determine what exactly a fixed magazine firearm is. After all most magazines can be removed, including tubular magazines and many similar magazines. In fact on some shotguns or rifles you can unscrew the magazine tube (without a tool even.)
To create a definition to define the difference between detachable and fixed magazine they said that a fixed magazine required a tool to remove to differentiate it with a magazine that you merely press a button or lever with your finger to remove.
The result was after some years with that ruling in place someone figured out that if you took firearms with features that would normally be prohibited and replaced the magazine release with a modified type that could only be pressed by something other than a finger it technically required a tool and since it was then a fixed magazine firearm it was not subject to the assault weapon features that only apply to firearms that have detachable magazines.
(However fixed magazine firearms can not have over 10 round capacities, even if grandfathered magazines are used they would be illegal assault weapons, while legal detachable magazine firearms that comply be feature can use grandfathered magazines.)
The bullet button was born, and CA residents once again had the large number of common firearms available to them again.
The legislator is closing this now with legislation that needs the governor's signature at this point to become law.
Unlike Tactikel said it was not created as a requirement to slow you directly, even though that may be the actual result. Rather it was a clever way an entrepreneur came up with to circumvent restrictions that kept Californians from owning most modern firearms that typically use box magazines as they were subject to restrictions similar to the expired Federal AWB.
AR type firearms for example almost universally have a pistol grip (except for those created for CA exemption purposes) and thus with a detachable magazine AR firearms would not be purchasable.
A bullet button allows the firearm to have a pistol grip, while also having a magazine you can still remove to load. Prior to that you were stuck with mini 14s, m1As and similar firearms without pistol grips if you wanted a removable magazine.
Likewise the Thompson you got would have been unavailable, as the market would not have them with magazines which could not be removed, nor would people be likely to purchase them with funky stocks lacking a pistol grip as they would no longer even look like a Tommy Gun ( and that iconic and recognizable look is why most purchase them) and so they would simply be unavailable.
There is other features as well that are opened up, but the hardest to work around had been the pistol grip.
