They call her "far-right" because that's what she is. The party is called "Front National", after all – "Front" as in war. They don't mind the label. It's simply a description of fact, with the possible caveat that some of their economic ideas aren't that different from those on the far left.
The media also regularly labels the other candidates, and nobody is complaining about them any more than Clinton complained when someone said she was left of Trump, politically.
Example from the NYT: https://www.nytimes.com/2017/04/23/world/europe/emmanuel-mac...
"[...]Emmanuel Macron and Marine Le Pen to go to a runoff to determine the next president, official returns showed. One is a political novice, the other a far-right firebrand "
"Mr. Macron, a former investment banker, abandoned traditional parties a year ago to form his own movement with an eclectic blend of left and right policies."
"[...] the mainstream right candidate François Fillon had nearly 20 percent, and the far-left candidate Jean-Luc Mélenchon had 19.6 percent."
As to why she is labeled "extreme", Wikipedia says:
- She favours a "radical change of politics in order to drastically reduce upstream the influx of illegal immigrants towards France"
- She supported a referendum on whether to reinstate capital punishment in France
- "But I openly admit that, to some extent, I admire Vladimir Putin."
- "I do not believe that there was an illegal annexation: there was a referendum, the citizens of Crimea wanted to join Russia."
- Denouncing "the US supremacy", she "refused the idea that France slavishly followed the USA in this new stalemate"