Well, salaries ARE taxed, actually. :

It's called the payroll tax, and yes it affects companies as much as employees; that's where funding for unemployment insurance comes from, after all.
Also, it's not just companies. Individuals can deduct advertising costs, too.
http://www.irs.gov/publications/p535/ch11.html#en_US_2011_publink1000209177
For that matter, explain to me why taxing only on profit and not on revenue is any different than a tax subsidy? Wouldn't it be nice if it worked that way for individuals? It also doesn't change the fact that the theoretically taxable revenue spent on advertising would dwarf the Congressional appropriation for CPB.
If true, why isn't WCRB/WGBH Classical advertising and raking in all that corporate welfare? Why haven't all the NPR talk stations relocated above 92.1 to do the same?
Well, first of all because it's not the STATION getting the welfare. It's the companies doing the advertising. Any NCE that magically turned for-profit and had a non-reserved-band (aka "Commercial radio") license would be competing for the same ad dollars that all the existing commercial radio stations would.
Second, by not being a non-commercial licensee, donations to the station would no longer be tax-deductible, removing a major driver of donations to the station.
Third, because you can't just magically move to the commercial band. It requires an allocation to be created (and said allocation has to be technically possible to fit in a given area) and then the license is awarded via auction. Let's pretend for a moment that WGBH could minor-change I.F. hop up 10.6MHz to 100.3MHz. They can't, but we'll pretend they can. Even though they're not in the reserved band (87.9 to 91.9MHz), they're still a non-commercial license. If they were to attempt to become commercial, they'd have to petition the FCC for a new allocation, surrender their existing license, wait for the auction to happen, attempt to outbid (yeah right!) the hordes of other entities that would kill for a new Class B in the middle of Boston, and then file a CP.