Looks like parport_pc driver ignores nACK line state and checks only BUSY line. 

There is this text it IEEE1284-2000:

Compatibility Mode forward data transfer phase: Begins when the host asserts nStrobe and ends following data hold time and nStrobe de-assertion. (Note that the host is not free to send the next data byte until the peripheral acknowledges the transfer using nAck.) The host may not initiate negotiation to a new
operating mode until the interface returns to Compatibility Mode forward idle phase.

I've set SPP mode in BIOS, Linux detected it:

user@host~$ cat /proc/sys/dev/parport/parport0/modes 
PCSPP,TRISTATE

NACK and BUSY is pulled up with port internal resistor to +5V, when I'm pulling down BUSY input to GND of LPT port, and trying to write there something (for example cat /dev/zero > /dev/lp0) there are no nACK lock ("successful" writing of any data without acknowledge from device). I've tried to read sources and haven't found any nACK checks/interrupts, looks like parport_ieee1284_write_compat() do not checks it, but it should.