You are right that GPS in general is easy to jam, and somewhat more difficult to spoof. However for LAAS jamming seems the only worthwhile method as for spoofing you need to not only spoof the GPS-correction values but also the geometrical approach info for each channel (instead of a frequency you dial in a 5 digit channel number on your NAV) separately as well as the checksums which are there to provide integrity monitoring, in fact you need a second correctly programmed LAAS with a higher power output than the original one. By the way, one LAAS can of course provide several airports at once with high precision approaches, although i do not know of any actual installation of that kind as even one per airport is much much cheaper than one ILS for each runway direction.
SBAS is easier to spoof as you only need to spoof the correction values and a very low power output from the WAAS or EGNOS satellite.
It will be interesting to see if jamming or spoofing will become a real thread anytime soon like it used to be with frequency jamming or spoofing for ATC frequencies some time ago. At the moment we still have the full backup of real ILS installations parallel to GLS plus other non precision approaches, not to mention that onboard monitoring of positions based on DME/DME and IRU information vs GPS information is done as well.