"Do musicians need to play these venues if they don't like the terms?"

Short answer is "No" but if there's nowhere else available and they need to gain exposure it leaves them in a bit of a predicament.

"Paul McCartney........is there something he can do about it?"

I wasn't sure where he fitted in either but I think it's just because he's such a high profile musician. His support could add a great deal of "clout" to the campaign. Also, The Beatles cut their teeth playing in small venues.

I'm not sure why he hasn't been contacted directly. There must be some way and I don't think he'll be that elusive.

Of course, other arists could also be approached.