Shouldn’t musicians just do it for the love?

Absolutely.. sometimes! It depends on the situation:

• If everybody at the venue-caterers, bartenders, the venue owner, the janitor - everybody - is donating their services, it’s all love, baby, perform your heart out. 
• If, however, you’re expected to do it “for the love,” or, for “exposure,” when others like the venue owner, booking agent, sound and door personnel, bartender, server, bar-back, and janitor are all being paid, that is not love. You are getting screwed.  
• Yes, this is even true for "Benefit" Concerts. We all want to give back to the community, but is it worth devaluing yourself, your craft, your brand, end every other musician in your town? Why are musicians the only ones expected to donate services? - Benefit concerts, by the way, are the least efficient way to make money. - It can still be a benefit if musicians are being paid. In fact, some of the best paying shows are for charities.  
• If you’re still confused, see this excellent flow-chart.
• see also Four Things Every Musician's Gotta Know.