yes its legal
The reason why some choose not to advertise sports book affiliate links is due to Google. Google is the worlds largest sand box and essentially makes all the rules w/regard to search.
In order to rank a page (make it to page one) in Google search a web page must prove that it offers value to the consumer.
IE: in order to rank a web page, webmasters will create back links to other content, spin articles for duplicate but slightly different content and auto-blog blog content across thousands of meaningless blogs with links back to the web page ... These and other black-hat practices were once widely used to compete for SERP (search engine results page) rank.
Google monitors its sand box with algorithms to distinguish who and what is deemed valid/good content. Back in 2011 Google decided that it was going to clean up their sand box and released an algorithm named Panda, which looked for sites who offer thin content, high ad to content ratios, Gambling sites and other factors .... Many sites not just gambling/porn sites were penalized and suffered by not ranking and essentially being buried. Google releases new algorithms every 3-6 months with new search criteria rules.
So as a result, many people have decided to not piss off the 900 lb gorilla, and instead either play by the rules or distance themselves entirely from the bad neighbor which is porn/gambling/payday loans etc etc.
However there are ways to rank a page legitimately within the Handicapping space still, the people who I am talking about who were penalized were fly-by night'ers who typically build a page, rank it , make a few bucks and then sell it quick before the next algorithm change comes.
If someone wanted to sell picks and understood what was necessary to build authority in the space, cultivate a following of people who considered your Handicapping worthwhile, and learned e-mail list building/marketing skills, then yes you make a lot of money selling picks, and running affiliate marketing.
But its a lot of work.
However all you need is a steady stream of people clicking through your sports book links and now your cooking with gas, especially if a whale deposits and loses a LOT.