In general, just because someone is providing a service to you (e.g. driving you, providing customer support, etc) doesn't mean they should be unnecessarily exposed to offensive language.
You do see their first/given name for identification on pickup.
They are as dumb and undesirable as incomplete regex email filters in web forms.
Every single one ever written will have this problem. And ironically, absolutely nobody reading a fart joke will ever be offended. But the millions of people who are denied service and not twitter famous enough to reach a dot-com employee are very much offended.
...Also, the single reason to add those auto-censorship features is to please advertisers, not to protect users/drivers from fart jokes.
Expect Advertisement on lyft very, very soon.