prepositions - Why do we have to use 'to' instead of 'will'?

I saw the sentence like

Donald Trump to run for president next month.

in the newspaper on the internet.

But I don't know why we have to use to instead of will.

Is there any rule I don't know?

Can anyone tell me the reason why we should use to in the news?

Answer

With the future, the infinitive form expresses intention whereas will + can express either an intention or the inevitable.

Consider this scenario:

Thousands of scientists worldwide agree that the polar ice cap will melt, based on their careful observations, unless measures are taken to dramatically reduce emissions of so-called greenhouse gases.

How might the headline read?

POLAR ICE-CAP TO MELT

POLAR ICE-CAP WILL MELT

Does the polar-ice cap intend to melt? No. So the verb would be will.

But let's say a government plans to cut the budget of its environmental watchdog agency. That is their intention.

GOVT TO CUT BUDGET OF ENVIRONMENTAL AGENCY

GOVT WILL CUT BUDGET OF ENVIRONMENTAL AGENCY

There, the choice is less clear. Probably the infinitive, but not necessarily.