I completely agree with this . I myself have had the issue , with even sites like Neteller freezing my account after I tried the withdrawal or deposited a large amount . They are never seen blocking before a deposit, but right after one they do it.
Some sites claim that its for your own good and for them to stay in line with the legalities, but thats pretty much a BS excuse for it . I haven't ever had the problem with bitcoin gambling sites though , so thats another reason they are much better than Fiat which requires ID and all.
It's to prevent money laundering. In order to comply with their licenses, gambling sites and payment processors are required to get KYC data from you as soon as you go over certain withdrawal thresholds - if they don't they risk losing those licenses, costing them tens of thousands of dollars (as well as risking prosecutions). Bitcoin gambling sites don't need to do it because for the most part they aren't licensed, and those that are are probably licensed in jurisdictions where bitcoin isn't subject to money laundering laws.
