As well as preventing people entering Palestine legally, they also use brutality to suppress peaceful protest.  And Governments and Airlines do nothing but the Israelis bidding:

http://www.haaretz.com/news/diplomacy-defense/video-senior-idf-officer-beats-pro-palestinian-activist-with-rifle-1.424454

http://www.haaretz.com/news/diplomacy-defense/israel-blocks-entry-to-first-pro-palestinian-fly-in-activists-hundreds-more-expected-1.424321 

http://www.morningstaronline.co.uk/index.php/content/view/full/117864

http://www.bbc.co.uk/news/uk-england-manchester-17720326

http://mondoweiss.net/2012/04/we-have-cancelled-your-booking-the-criminalization-of-travel-to-the-west-bank-is-laid-bare-to-the-world.html?utm_source=Mondoweiss+List&utm_campaign=5f08d9b8b9-RSS_EMAIL_CAMPAIGN&utm_medium=email#comments 

http://www.guardian.co.uk/world/2012/apr/15/israel-blocks-entry-pro-palestinian-activists

http://www.youtube.com/watch?v=ebL5Woe8aCA&feature=share

http://rt.com/news/palestine-israel-activists-letter-081/