Again a bad example from a performance perspective.
1) Using a bigint vice tinyint in this case requires 7 extra bytes of storage and 7 extra bytes to chew through on row accessed. And yes, those 7 bytes DO make a difference!
2) Prohibiting improper enum usage would require under-the-covers system validation, also slowing performance for every query that references an enum. Once again a developer 'convenience' equals reduced throughput. Consider also what happens if you change an enum or change which enum a table references.
Microsoft has for many years gotten a bad name in the server business (especially database) for sub-optimal performance. A lot of that came because they made things REALLY, REALLY EASY for developers. RAD this, point-and-click program that. Like parameter refresh, etc. But easy for developers quite often turns out to be a performance DOG when any reasonable amount of data is thrown at the application.
I guess I shouldn't complain too vigorously however, since I make a comfortable living fixing other people's bad designs, bad data structures and bad code!! :-))
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru at GMail