Tharsis have 2 passive bonus, when a city is placed on mars, gain mc production.
When you play a city anywhere, gain 3mc.
The game have several cards that interact with cities and a find odd this variable between all cities and only cities on mars. (There is 2 possible cities outside of mars, ganimedes and phobos.)
all the cards with bonus from cities
Pets city anywhere
Immigration city anywhere
Energy (something) anywhere
Zepelins only mars
Martians trails only mars
By the theme of the cards, they make sense and stuff, but i would be really happy with some more streamlining of this... And i also noticed people pretty often misplaying the first passive from tharsis. Well those are my 2 cents, i love the game, just find this a bit odd.
There are also expansion(s?) coming with cities to be built on other places, so it makes sense both as you say with the thematics of those cards and from a balancing factor. Also, I think that it is great with some variation, but true as you say that some people might miss the restriction. You better be observant when reading the cards. Another common mistake is to believe that "Mining area" and "Mining rights" is played the same way, but they aren't.