Bal Tosif in context

Summary: It relates to changes in sacrificial worship. How can adding positive things turn out negative? Shadal gives an example.

Post: While the Christians, who established the perakim, placed this pasuk:

א  אֵת כָּל-הַדָּבָר, אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם--אֹתוֹ תִשְׁמְרוּ, לַעֲשׂוֹת:  לֹא-תֹסֵף עָלָיו, וְלֹא תִגְרַע מִמֶּנּוּ.  {פ}

1 All this word which I command you, that shall ye observe to do; thou shalt not add thereto, nor diminish from it. {P}

at the start of perek 13, severing it from what preceded, according to Jewish tradition there is a gap, a petucha, designated above by { פ }, when severs it from what follows and links it to what precedes. That context is:

כט  כִּי-יַכְרִית ה אֱלֹהֶיךָ אֶת-הַגּוֹיִם, אֲשֶׁר אַתָּה בָא-שָׁמָּה לָרֶשֶׁת אוֹתָם--מִפָּנֶיךָ; וְיָרַשְׁתָּ אֹתָם, וְיָשַׁבְתָּ בְּאַרְצָם.	29 When the LORD thy God shall cut off the nations from before thee, whither thou goest in to dispossess them, and thou dispossessest them, and dwellest in their land;
ל  הִשָּׁמֶר לְךָ, פֶּן-תִּנָּקֵשׁ אַחֲרֵיהֶם, אַחֲרֵי, הִשָּׁמְדָם מִפָּנֶיךָ; וּפֶן-תִּדְרֹשׁ לֵאלֹהֵיהֶם לֵאמֹר, אֵיכָה יַעַבְדוּ הַגּוֹיִם הָאֵלֶּה אֶת-אֱלֹהֵיהֶם, וְאֶעֱשֶׂה-כֵּן, גַּם-אָנִי.	30 take heed to thyself that thou be not ensnared to follow them, after that they are destroyed from before thee; and that thou inquire not after their gods, saying: 'How used these nations to serve their gods? even so will I do likewise.'
לא  לֹא-תַעֲשֶׂה כֵן, לַה' אֱלֹהֶיךָ:  כִּי כָל-תּוֹעֲבַת ה אֲשֶׁר שָׂנֵא, עָשׂוּ לֵאלֹהֵיהֶם--כִּי גַם אֶת-בְּנֵיהֶם וְאֶת-בְּנֹתֵיהֶם, יִשְׂרְפוּ בָאֵשׁ לֵאלֹהֵיהֶם.	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 do they burn in the fire to their gods.

See my detailed discussion of this in my running commentary on parashat Vaeschanan. What is the extent of the two instances of bal tosif. It seems, on a peshat level, to be associated not with adding mitzvos in general but with modifying sacrificial practices.

At any rate, Shadal has an interesting explanation there in Vaeschanan about how, via repeated slight adjustments, positive law can be something awful. He says something similar here, about bal tosif:

 א ] את כל הדבר וכו ' לא תוסף עליו: בעשותך מצווה לכבוד ה ' אל תוסיף דבר מדעתך בחשבך שיהיה לרצון לפני הקל, כי לא כמחשבותיך מחשבותיו, כגון לא התירה התורה על גבי מזבח רק שור וכשב ועז ותור ויונה, ואם נוסיף להתיר גם הצבי והאיל, אולי יבאו אחרים ויתירו גם האדם.

"אֵת כָּל-הַדָּבָר ... לֹא-תֹסֵף עָלָיו -- when you do a mitzvah for the honor of Hashem, do not add something of your own innovating ideas, in believing that this will be received favorable before Hashem. For His thoughts are not as your thoughts. For instance, the Torah only permitted upon the alter an ox, sheep, goat, turtledove and dove. And if you add to permit as well the deer and the gazelle, perhaps others will come and permit human sacrifice as well."