literally nothing is stopping them from allowing local sessions offline or on lan, Battle.net online sessions would still be server side
real reason is monetisation, nonstop data gathering in a live service to tailor their monetisation, limiting the player's ability to go outside their monetisation scheme, ie, the same as D3 and RMAH
so no good reason that benefits anyone playing the game really, but "muh cheatorz" is a nice copout, not like D2 had closed B.net or anything, and it totally stopped people from cheating in D3 too
imagine getting banned from playing SP in D2