> He said he transferred it to his personal Dropbox cloud account to use later on his personal computer.

Some companies don't draw the line between personal and work computers.

Most schools, colleges, and universities expect students now to supply their own computers.

But, while it's not surprising that someone today could consider that it might be ok just to take the data home like it's no big deal, I think Tesla isn't making a mistake in taking this to court, if it was clear in the contract(s) that the employee signed that such behavior was unacceptable.

You could jump to conclusions reading the title that the employee planned to sell the data or was working for a competitor, and you could be correct. But what's at issue, it seems, is that the employee didn't meet their contractual obligation.