Why are prepositions and subordinate conjunctions grouped as the same tag in the Penn Treebank tag set?

Question

I was reading a book by Jurafsky et. al. It states following:

• English adpositions occur before nouns, hence are called prepositions. They can indicate spatial or temporal relations, whether literal (on it, before then, by the house) or metaphorical (on time, with gusto, beside herself), and relations like marking the agent in Hamlet was written by Shakespeare.
• Subordinating conjunctions are used when one of the elements has some embedded status. For example, the subordinating conjunction that in “I thought that you might like some milk” links the main clause I thought with the subordinate clause you might like some milk. This clause is called subordinate because this entire clause is the “content” of the main verb thought.

Then it gives following table:

Why prepositions and subordinating conjunctions are grouped into the same tag IN in Penn treebank tagset? Is there any linguistic consideration?

Answer 1

Look up here:

Complements of P PPs are headed by prepositions and take complements of various categories. The most common are NP and CP, but ADJP, ADVP, IP, PP, etc. are possible as well. Since subordinating conjunctions are treated as P in our corpora, subordinate clauses introduced by a subordinating conjunction are labelled PP. See below for examples. https://www.ling.upenn.edu/hist-corpora/annotation/syn-phrase.htm#pp_comp