It is only a presentation issue.
There's no such thing as "unlimited" in the end, since the max theoretical size
• for a data file is 32TB and
• for a log file is 2TB. 2,097,152 MB = 2 TB.
I.e., the value you see is the maximum theoretical size.
Although file size is displaying as "restricted", the MAXSIZE is indeed 2TB (2,097,152MB).
So, within the confines of capacity limitations, it is unlimited so to speak.
Working with tempdb in SQL Server 2005