Why is `restriction` a relation-symbol?

I'm not sure why amssymb defines restriction as a math relation symbol.

DeclareMathSymbol{upharpoonright} {mathrel}{AMSa}{"16}
 globalletrestrictionupharpoonright

Given these definitions, you can “fix” it according to your taste by making it an ordinary or binary operation symbol:

documentclass{article}
usepackage{amsmath,amssymb}

DeclareMathSymbol{ORDrestriction}{mathord}{AMSa}{"16}
DeclareMathSymbol{BINrestriction}{mathbin}{AMSa}{"16}


begin{document}

$frestriction A$ (relation)

$fORDrestriction A$ (ordinary)

$fBINrestriction A$ (binary operation)

end{document}

Once you have made your choice, you can remove the undesired definition and also the capitalized prefix.

If you want the relation spacing but also to avoid line breaks after the symbol, you can do

renewcommand{restriction}{upharpoonrightnolinebreak}

If you prefer cargo cult programming

renewcommand{restriction}{upharpoonrightnobreak}