There are several reasons for the WMF to be politically active in this way. Firstly, it reflects Katherine Maher's personal views, which seem to be somewhere around the Democratic Socialist point on the spectrum. Secondly, it is popular with the staff and thirdly it is popular with the volunteers -- I group these together because they tend to come from a similar demographic, and indeed the sinecure jobs at WMF are there to reward particuarly loyal cult members. Fourthly, it is popular with the big Silicon Valley companies, some of which are among the big donors. Of course the big employers want the USA to keep sucking in trained technical people from around the world -- it keeps up the supply and keeps down the price, and gives them a competitive edge over their international rivals. Because there people are educated at the expense of other, poorer, countries, it keeps down the taxes in the US that these big employers occasionally deign to pay. So it's entirely rational for Katherine to position the WMF in this way, and it certainly doesn't do her any harm either. After all, she's spending someone else's money.
I noticed an interesting point about the blog post, by the way. It mentions "disturbing comments made by the current administration about the religious basis for some of these restrictions" and references Al Jazeera
. Passing over whether that's a Reliable Source in the jargon, we see that the story is dated March 2016 and explicitly states "Republican presidential frontrunner Donald Trump has called for a "total and complete" block on Muslims entering the United States." Well, that is disturbing, but it's simply untrue to ascribe that to the "current administration": it is one of many primary-era campaign speeches not a statement of current policy. Presumably such minor matters as accuracy pale into insignficance compared to the greater cause of using the donors' dollars to propagate The Truth.