What might be the reason for that?
I guess, the "Latin 2" vs. "Latin 1" unfair division during early computer age, before Unicode revolution, is part of it. In those times Swedes could easily communicate with French (Latin 1) but not with Poles (Latin 2). But some people, including Polish users, do not know how to take advantage of Unicode and stick to old software. I still see some web pages originating from Poland, which are rendered improperly, so instead of "Wałęsa" a reader sees somthing like "Wa%#sa".
I always see Polish surnames spelled without Polish letters: ą, ę, ł, ż, ź, ć, ń, ś,
The "always" part is not correct. Enter Wałęsa in Google's search field and try news, English results only, in order to eliminate Polish sources. I just did it - one of the top results is this:
Edit - not this one
What the West Gets Wrong About Belarus
Central Europe Digest - Edward Lucas - 2 May 2011
None of the opposition leaders has emerged as a charismatic, credible leader similar to Lech Wałęsa or Václav Havel. It is true that they faced a difficult ...
But statistically - you are right. Edward Lucas (above) is a good friend of Poland, and he knows the correct spelling.