Why many research results are presented as paid articles? Many times I learn or look for some information, I find scientific article on ieeexplore.ieee.org or similar, where articles are paid. Most of the articles are written by some professors of universities or students about their work.

My question is, why do they sell their results?

If I was a professor and made some work, I would like to gain some acknowledgement for it and I think there is no better way then see others work based on mine. So why not to publicly share the article for free? 

I understand that private companies sell their innovations because that's the main source of their income, but people at universities are usually paid and writing articles are part of their job.

So what are the advantages of selling results instead of sharing for free (is there any other except money)?  

Does selling your results have any negative influence?

PS: I may not have enough backstage knowledge about how it works on universities and I don't want to offend anybody. I also don't mean work people do in their own free time, but work they do on behalf of universities. This just slows down the development and overall research in field in my eyes.
Professors who publish their research results normally do not earn money from these paid articles. It is the publishing journal who earns. There are even journals who ask the author to pay for publishing the articles.

Contact the authors of the paid article directly and ask them for the copy of the article. Be polite, explain why do you need it for. In most cases, you will get that copy for free.

Also, try put the article name and authors into Google search. Another similar article by the same authors may exist and be free to download.