Volkswagen Tiguan 2009: Will I lose my warranty if I service it myself?

I'm a qualified motor mechanic, engine reconditioner and aircraft maintenance engineer, and I am bemused that my Volkswagen dealer tells me that if I carry out any maintenance on my 2009 VW Tiguan TDI I risk voiding my warranty. Reading the maintenance schedule, apart from changing oil and filter the rest is a series of visual inspections that I believe am more than qualified to perform. I believe that 15,000 km or one year between services as recommended by VW is too long, especially on a diesel engine driven mainly in the city, and I would like to change oil and filter myself at 7500 km. The dealer is more than happy to change the oil and filter at the reduced interval, but I refuse to pay a ridiculous amount of money to carry this out. Why can't I do it myself?