Why is NASA still giving away flights on its own vomit comet?

http://www.lamar.edu/newsevents/news/207_4470.htm

"Students from Lamar University will experience weightlessness on
flights aboard NASA¹s Weightless Wonder this week as part of the space
agency¹s Reduced Gravity Student Flight Opportunity Program."

If NASA feels the need to spend taxpayer money on giving joyrides to
students, why must it still use its own airplane for doing so? Why
can't they simply pay Zero Gravity to fly them instead -- accomplishing
the same goal (good PR, inspiring students, whatever it may be) while
also supporting the commercial sector?

Indeed, why is NASA still using its own vomit comet for ANY purposes?
Isn't there law in effect that requires it to purchase equivalent
commercial services where available, rather than relying on its own
craft? Or does that only apply to spacecraft?
.

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