They were not your average Pole. They were supporters of the ruling party in Poland, Law and Justice Party (PiS), who are allies of Trump, and they were bused in from all over Poland.
Some are making it look like Trump was being given a hero's welcome, but it was more like a fake staged welcome for a Communist tyrant from the Soviet era.
It's all about increasing NATO presence in Poland and flogging missiles.
...But while Trump basked in the applause, there was a lot of effort behind the scenes by the ruling Law and Justice Party (PiS) to make sure it happened, which isn’t necessarily easy in a country where only about 23% of the Polish public is confident in the US president.
Polish leaders are old hands at this sort of thing, however, with the PiS busing its own party supporters from around the country into Warsaw for the occasion as a “great patriotic picnic.” The tactic immediate draw comparisons to Cold War-era welcomes for visiting Soviet officials.
...Getting NATO troops permanently stationed in Poland has been a priority for governments of all stripes there, and Trump may be the path of least resistance for the PiS, with Trump’s government eager to prove support for NATO and Poland an easy target to throw that support at, particularly so long as they can make with the cheering crowds, and sign lucrative missile deals for US arms dealers.
[source : Polish Govt Manufactures a Warm Welcome for Trump, Antiwar, http://news.antiwar.com/2017/07/06/polish-govt-manufactures-a-warm-welcome-for-trump/, 6th July 2017]