Maybe it's a stupid question (If it is I apologize for this) but I don't
understand why some 2600 programs (e.g. Combat) are using the SEI
instruction at the begining of their code ? As far as I know the 6507 has no
interrupt inputs.
Although the 6507 doesn't have NMI or IRQ pins, it's good programming
practice to have those vectors point somewhere, either the same as RESET or
an RTI instruction. (And to include an SEI instruction in your
initialization section.) That way if the 6507 flakes out it won't crash
horribly.
As well, the 6502 used in the 7800 does have NMI and IRQ, so it's not
inconcievable that they could be triggered.
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