Just because they don't stop their competition from entering the market would not change whether or not something is a monopoly. They have 88% of the US search market share which is often consider to be in the monopoly territory.
Of course it may not be prudent to go after a company that isn't doing any malicious against its competitors.
How would a government even go after a monopoly that isn't doing anything malicious against its competitors?
What would the punishment even be? Forcing adoption of other platforms to artificially inflate usage rates? Breaking Google Search into two competing Google Mini-Search products that aren't expected to cooperate and push search #3 and onward out of the market?
Yelp is a parasite that participates in extortion of small businesses. They need to be investigated themselves long before their word has any weight in this matter.
Of course it may not be prudent to go after a company that isn't doing any malicious against its competitors.