Whatever it may be, it's not theft. Stealing something entails depriving someone of that thing.
The morality and the rights and wrongs of it are another matter altogether, but it is not now, and won't ever be, theft.
WRT to the depriving Garmin of their money - maybe if they didn't hobble the product with irritating DRM and charge an outrageous fee for it (£200 stand alone versus £10 when bought with hardware) people might not seek out pirate versions of the maps.
Time and time again, it's been demonstrated that people will pay for stuff when it a. represents value and b. when the product purchased isn't stupidly limited, and doesn't operate on the principle that you're fundamentally dishonest.
Lots of people now happily pay for music again, but weren't so keen back in the day when the record industry tried to force a "buy the music for each device you play it on - 5 devices, pay 5 times" model (lovingly backed by the halfwit Rundgren).
Ironically, it's the music pirates you have to thank for the DRM free mp3 purchases you enjoy today - with no SDMI or CD based malware kicking around (see Sony as a prime example of targeting paying customers and treating them like criminals by illegally installing hardware breaking malware on peoples PC's)