Dr. Bullinger sheds some light here...
offered = brought near. Hebrew. karab. App-43.
strange fire: i.e. fire other than that Jehovah had commanded, required, and given from heaven (Lev_1:7; Lev_6:12; Lev_9:24; Lev_16:12. Compare Exo_30:9). It was of their own kindling: so is all that is offered to God in worship today. If so, according to Joh_4:23, Joh_4:24, it is "strange fire", and deserves the same judgment! All worship that is not kindled by the Holy Spirit is "strange", and is of the flesh. Compare Joh_3:6 with Joh_6:63. It "profiteth nothing", and "God has no respect to it" (Gen_4:4, Gen_4:5. Heb_11:4). The incense of prayer and worship on the golden altar in the holy place was kindled by fire taken from the brazen altar in the outer court, on which atonement was made (see Lev_16:12, Lev_16:13 and Rev_8:5): only those, therefore, whose sin is atoned for can worship. Compare the "strange incense", Exo_30:9.
commanded them not. Negative. The introduction of anything "strange", where all is ordered by God, is abomination in His sight; and calls for, and calls down, His judgment. Thus the first recorded individual use of incense began in disobedience (Lev_10:1), and the last ended in unbelief (Luk_1:10, Luk_1:18, Luk_1:20).
Now read this...
Deu 12:29 When the LORD thy God shall cut off the nations from before thee, whither thou goest to possess them, and thou succeedest them, and dwellest in their land;
Deu 12:30 Take heed to thyself that thou be not snared by following them, after that they be destroyed from before thee; and that thou enquire not after their gods, saying, How did these nations serve their gods? even so will I do likewise.
Deu 12:31 Thou shalt not do so unto the LORD thy God: for every abomination to the LORD, which he hateth, have they done unto their gods; for even their sons and their daughters they have burnt in the fire to their gods.
Deu 12:32 What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.
They thought it didn't matter, that God would be happy that they were worshipping him. After all, haven't you heard the song...
"Just as I am?"
God doesn't accept us just as we are and He certainly doesn't accept any form of worship except the type He has specifically laid out.