To say "I will pray for you" is different from saying "I will keep you in my thoughts".
It's saying they will ask some higher power to come to your aid.
It is implying help will come, while saying they won't help you themselves.
Now, maybe they don't purposefully intend that message ... I'm sure they don't.
But when looked at objectively, that IS the message.
It is also predatory / abusive in nature.
It implies - and some people will state it outright - that if you are NOT helped, it is somehow your fault.
You didn't have enough faith - or - You must deserve it - or - You are going through a trial.
In other words, they blame the victim.
I'm not transferring the "blame the victim" mentality.
Nor am I saying everyone that says "I'll pray for you" believe that.
However ... having grown up in the church and been around believers all my life ... I can tell you there are literally thousands of people who will actually tell people "You didn't have enough faith" or otherwise voice their blame of the victim. And there are very few people in the church who will refute it, when it's said.