You sure ask a lot of questions for somebody from Maryland.
The SS can jam frequencies ...but so can just about anybody else with off the shelf equipment.
What do they wish to jam?
There are at least 6 different regularly used commercial cellular bands or blocks of frequencies used in North America. More worldwide.
Unless you know the target frequency of your subject/ device, you will have to jam the entire band or block of frequencies. So if I'm concerned with an IED using a GSM, TDMA, CDMA trigger...then you need to jam the entire suspected frequency range . You need to block that trigger from hearing a signal inside the target area.
If the secret service deployed a jammer that covered every Verizion, Sprint, AT&T user inside a 3 square mile radius... the FCC would shit bricks the size of Maryland.
They would still shit bricks if you knocked out every user within a 500ft radius. You need a lot of wattage to kill signals like that, and only the Military does that kind of stuff during wars.
In the world of jamming that's "scorched earth" stuff.
But...
There are off the shelf Cell Jammers available that target much smaller areas under 100 feet, depending on conditions. So if you suspect that someone has hidden a device triggered by a signal in one of the traditional cellular bands...then it's no big deal to jam and knock out that band inside a 50 foot radius.
Everyone inside that protected 50ft radius is presumably already screened and under your control...and the Secret Service has a pretty secure/ encrypted 400mhz spread spectrum radio system that is used inside the bubble(within 25ft) of protection.
Target+Range= Power
It's likely the jammers deployed are low power and very transiant in use inside the USA.
Outside the USA is a different story....
Entire Cell systems and bands sometimes go dark during Presidential visits in 2nd and 3rd world countries.
Regular members of the POTUS press pool report mysterious cell "blackouts" in places like Brazil, Vietnam, and most of the African countries. The blackouts are annoying but usually short in duration.. It's pretty much understood that it's the Secret Service radio techs testing the limits of the jammers outside the view of the FCC...and on unsuspecting 3rd world countries. (win win).
So yes...jamming happens...but in the USA it's limited and highly directed in nature if deployed....to limit interference to our domestic cellular networks. Any other larger deployment in the USA would just piss-off tons of cell users including public safety etc..
You can get away with landing Air Force One on the 405 freeway in the middle of rush hour...but if you Jam the Facebookers, Tweeters and Fapchatters from posting the pics, then you will have bloody hell to pay.