I don't think it should be a problem for Pep to speak to opposing players after a game so long as what he says is not inappropriate. Players can and do talk to one another. FFS, these people are, 99% of time, adults of voting age.
What is questionable is the manner in which Pep expressed himself to Redmond. However, he was just being emotional and was not taunting or attempting to physically harm one of his opponents (unlike, for example, Lukaku or PEG). Also, Pep's team had won so this was not some case of sour grapes.
Pep is a demonstrative, tactile guy. I wonder if there would be so much fuss if this incident had happened in Spain? Are cultural differences a factor here?
My own view is that Pep should not have been so aggressive and demonstrative in his manner but I do not think his comportment in this instance was a punishable offence. As noted by other, what Pep has done here pales into insignificance when compared to Wenger's comments about Sterling. I'm not sure the FA (or the media) understand the concept of substance over form.