Here's the thing that is sort of being mentioned, but not fully.
Brand recognition is the MOST important thing to these companies. If they think you'll buy something that supports Atmos, then they'll market it this way. Also, they aren't misleading anyone. The soundbar does support Atmos, just not well or fully. For someone to think a true Atmos setup can be replicated by a soundbar...well? I can't believe they even expected it to. Supporting a specific audio codec in no way means that whatever device that supports it will do it justice.
It's sort of like the folks that buy Bose 5.1 cube systems and think they're fantastic...until they hear a real sound system. Bose still supports Dolby Digital/TrueHD and the DTS variants. Is it up to Dolby and DTS to force companies to make quality products? I wish it were, trust me, but this is marketing we're talking about, not actual performance.
No company in their right mind will advertise a soundbar and say "well, it's OK, but you really should go with a true Atmos install". Not happening. Doesn't matter how true it is.
Also, if you've ever sold A/V at any point, adding lite to the end of something will not make people think it's different. People would have to read in order to see it.