There are many reasons why fares could be different but looking for LAX-AMS and AMS-LAX return flights in the coming month on ITA's Matrix, the fare for the cheapest flights seems roughly equivalent in both directions, around $850-$900 so I don't think that what you heard is true (if you want to reproduce this or fiddle with the parameters yourself, I was looking for return flights with a 7-day stay, with Los Angeles as “sales city”).
Taking one random flight, LAX-AMS with Delta and KLM on May 4, returning on May 12, I see that the fares themselves are indeed only $760 for a total price of $1373. But a big chunk of the difference is accounted for by something called “DL YR surcharge”, which is not a tax at all but a surcharge imposed by the airline itself.
Here is a full list of all taxes and government-imposed fees according to the website:
US International Departure Tax (US) US$17.70
US September 11th Security Fee (AY) US$5.60
US Passenger Facility Charge (XF) US$9.00
USDA APHIS Fee (XA) US$5.00
US Immigration Fee (XY) US$7.00
US Customs Fee (YC) US$5.50
The Netherlands Passenger Service Charge (RN) US$15.20
The Netherlands Domestic and International Noise Surcharge (VV) US$0.50
Netherlands CJ (CJ) US$12.90
US International Arrival Tax (US) US$17.7
US taxes and fees are higher than Dutch taxes ($67.50 vs. $28.60) but very far from $700 (total of all taxes: $96.10). All or at least most of them (an in particular the $17.70 departure/arrival tax) apply equally to flights originating out of the country, foreign airlines and flights booked abroad.
Now, airlines often split the price of their tickets in “airfare” on the one hand and “taxes and fees” on the other hand. But that's merely a trick they use to make the fares look lower and go around system limitations or the rules of their own loyalty programs because the largest of these fees (like this “DL YR surcharge” and other “fuel surcharges”) are entirely under their control. That's probably the source of the confusion (which is certainly encouraged by the airlines themselves!).