Taking money off the table.

[AlexHoops]

The last time I was playing in a casino I was at the 10/20 LHE table. I showed up with around $500 and built that up to around $700. A new guy sits down next to me and he buys $150 in chips off of me. Without thinking I make a motion to put the money in my wallet and the whole table freaks out. They told me I needed to keep the cash on the table. I should have asked the dealer for the actual ruling on this but I didn't. I just kept the cash on the table and then asked them why I had to, it's a limit game. I could have bought in for a million if I wanted, or I could get away with $240.

At the tables online I notice the rebuy option, but the the cashout a portion of your stack option, so I assume there really is a rule about this. Why?