Even though its just news that Tesla bought a large amount of Bitcoin, their stock value is still dropping and they are trying to link it with the purchase of Bitcoin. Nobody knows for sure why Tesla bought Bitcoin but its Elon's decision and if he wants to advance his company with the use of Cryptocurrencies, he can do it. Its also a big move for Tesla because other car companies are trying to follow with integrating their systems with the use of Bitcoin and other Cryptocurrencies. This just means that what he did affected the lots of industries and its great for Cryptocurrencies because they are having that credibility that they needed to boast publicly.
I checked the TSLA stock price and it looks stable to me. Ever since January 2020, the prices have risen by almost 800% (more than that of Bitcoin). And I don't think that they can link any potential downside with Bitcoin purchase (I haven't heard any such statement from their officials). Because the Bitcoin purchase was hugely beneficial for TSLA. They purchased BTC worth $1.5 billion two months back and now they are having an unrealized profit of $900 million. This should actually pump their stock, not cause a decline.