Some ranges are set aside for GPS/GNSS: https://gssc.esa.int/navipedia/index.php?title=GNSS_signal

Some are monitored by the SMAP satellite: https://en.wikipedia.org/wiki/Soil_Moisture_Active_Passive

(Also in the L-band are all sorts of other things. A lot of cellular bands, known in the US as PCS and AWS, and more. Several different sat-phone systems. ADSB in 1090 is juuuust inside L-band. And more...)

All transmitters produce a bit of out-of-band interference, beyond where they meant to transmit. This is filtered to reduce it to a certain level below the intentional frequency, but filters aren't perfect. So when someone tries to jam GPS or other services, they inevitably bleed some energy into neighboring allocations too, some of which seems to be being picked up by SMAP.

Note that SMAP's passive radiometer doesn't have pinpoint spatial resolution, 36km is stated. This means it's listening to a pretty significant patch of ground at any given time, so for a source to be picked up among all that, it's got to be pretty loud. It also means that attributing the source is limited in precision, you can get to city level but not city-block level from this data.