Tesla is headquartered in an ally of the EU; BYD isn't. Maybe Tesla's subsidies are a problem to the EC - I don't know. But you're looking at it in a slightly simple way, as if it's very important that this process needs to be fair.
It doesn't need to be fair; it needs to be good for the EU. Is it good for the EU to impose tariffs on subsidised Chinese vehicles coming in (if indeed they are subsidised)? Quite possibly. (Quite possibly not: how important is it to have a big car manufacturing industry, versus your population having cheaper cars?) Whether it would also be good to impose tariffs on Tesla vehicles is also a valid question to ask, but those questions don't have to have the same answer.