Do you feel Nexon exploits it's playerbase?

The players exploit themselves. Nexon just provides the means.

While not entirely apt, the best comparison I can think of is entrapment.
Nexon doesn't "just provide the means", but they also create the incentive.

Nexon isn't exploiting their customers if they was they would be forcing you to pay up but no its a free game, everything is free.

It's fake free. Basically, playing Maplestory without access to NX is like playing a demo.

I really wish people would stop using the phrase "no one is making you do it".
Exploitation has never, ever, ever required force.

MTS offers people to play this game without giving a cent to nexon. So how can that be exploiting?

Hai! Clearly you have no idea how MTS works.
The NX doesn't magically appear there.