The fixed wing world get their clearance on the stand.
Perhaps the majority of IFR clearances that you hear passed to fixed wing on stand are published SIDs ?
I would guess that when you depart rotary SPIFR you require a local IFR clearance, and that ATC issue this nearer to your departure because the most expeditious solution is often dependent on the traffic situation.
Not entirely satisfactory from your point of view, but it may partly explain why you appear to be treated differently.
When you need to set down on the taxiway to copy your clearance, tell ATC in advance, and they should accommodate this with good grace. If you continue to get grief then, as previously mentioned, I suggest you telephone the relevant ATSUs to discuss the matter.