Nothing abnormal about it.
It happens to be the county seat, county seats are good candidates for US District Courts, and most 'Merkins prefer government toadies to be as far away from them as possible.
As for it being the only industry in Tyler, given a population of 109,000+ and that none of their top 10 employers are law firms or lobbyists, you must be thinking of some other Tyler, Texas.
I'm not normally a fan of Wiki, but this is actually the sort of thing they are decent at: http://en.wikipedia.org/wiki/Tyler,_Texas